In Chapter 4, the piano in the first sanctuary allows for a much more expansive range of notes than other in-game pianos. Accordingly, Susie and Ralsei have special reactions if the player inputs specific songs on it:

• Playing "Memory" results in Ralsei complementing it by comparing it to a lullaby.
• Playing "Your Best Friend" from Undertale results in Ralsei calling it "friendly and sweet," alluding to Flowey's introductory cutscene in that game.
• Playing "The Legend" flatters Ralsei, to which Susie responds by joking that Kris is flirting with him.
• Playing "Lost Girl" results in Susie wondering if she could learn it on guitar.
• Playing "Rouxls Kaard" results in Ralsei recognizing it as Rouxls' theme song; Susie then jokingly calls it the "theme of him getting his ass kicked."
• Playing "Heartache" (Toriel's boss theme in Undertale) causes Ralsei to describe it as "emotional" before Susie suggests playing it for Toriel.
• Playing "Undertale" results in Susie deriding the song as "crap."
• Playing "Dark Sanctuary" results in Susie pointing out that "Ralsei was humming it earlier."
• Playing the song on the Old Man's sheet music results in different responses depending on whether or not the player includes the octave switch: omitting it results in Susie & Ralsei commenting on its absence, while including it results in Susie commenting on how different it is.
• Playing "Your Dad's Best Friend" causes the pair to comment on the "annoying" nature of the melody, alluding to Flowery's portrayal in Chapter 5 as obnoxiously heroic and the animus that Ralsei displays towards him.
• Playing the first four notes of "MEGALOVANIA" (Sans' boss theme from Undertale) results in the Annoying Dog abruptly running Kris over with the car from Cyber City's hidden alley in Chapter 2, which causes a smash to black before the game returns to normal. Attempting to replay the song results in Kris fumbling the fourth note.

Several other songs that produce reactions are transcribed unusually, and as such are more difficult to trigger if playing solely by ear:

• Playing "Don't Forget" (C, D, E, E, E, E, E, A, C, B, B, B, B, B, B, C, B, A, A, A, A, A, A, G, H, G, E, C) causes Susie to fall silent before calling the song "kinda cool."
• Playing "Spear of Justice" (E, D, B, C, A, B) results in Susie calling the song "majestic" before Ralsei suggests that the Old Man would enjoy it, referencing the song's nature as Spoiler:both Undyne and Gerson's leitmotif.
• Playing "Chopsticks" by Euphemia Allan, "Heart and Soul" by Frank Loesser & Hoagy Carmichael, or "You've Got a Friend in Me" by Randy Newman & Lyle Lovett (C, E, C, F, E, A, F, H, F, H, E) results in Susie scolding Kris for "messing around." Notably, all three of these are real-life, commercially released songs: the first two are common stock piano pieces, while the latter was recorded for the 1995 animated film Toy Story. Its inclusion likely references a series of comedic edits of the song posted to Tumblr by developer Toby Fox. One of these edits, "Friend inside Me", was previously the subject of tongue-in-cheek fan theories claiming that it would pertain to Chapter 3's hidden boss.

In the livestreamed playthrough of Chapter 1 conducted for Undertale's sixth anniversary, it was revealed that Lancer's childlike characterization was originally far more subdued. Toby Fox later chose to exaggerate these elements after playtesters instead assumed that Lancer was a Danny DeVito-esque adult.

Rouxls Kaard's line "Lost...? Frightened...? Confused...? GOOD!! HA HA HA!!!" appears to be a reference to the 1995 edutainment title I.M. Meen, in which the eponymous antagonist says a similar line.

In a retrospective post detailing the making of Chapters 3 and 4, director Toby Fox revealed that the character design of Tenna was closely inspired by Mr. Tv Dinnah from the game Little King's Story, while the unconventional 3D graphics in the game Ghost Trick: Phantom Detective inspired him to use pre-rendered sprites for Tenna. Fox first attempted to shoot motion capture rigs with a Kinect and then import them into the animation program MikuMikuDance. Later, after drawing concept art for the character in 2017, he attempted to have Kanotynes create a 3D model for him, though this fell through for unspecified reasons. The final model for Tenna's sprites was ultimately put together by PixelatedCrown based on Gigi DG's interpretation of Fox's artwork.

After Kris exits the bathroom during Chapter 2's epilogue, the interior of the Dreemurr household noticeably changes: the phone and the painting by the trash can are both moved further up the wall, and the refrigerator grows noticeably taller.

These changes are also present in a mockup of the interior found in the game's files. However, the mockup is set during the day instead of at night, and the painting beside Toriel's chair is moved upward. This implies that the nighttime version of the interior was based on the mockup (with only minor touch-ups) without cross-referencing the layout seen earlier in the game.

In a retrospective article about the development of Chapter 5, developer Toby Fox stated that Flowery was inspired by a common misspelling of Flowey's name among Undertale fans in the months following that game's release. This is reflected in early concept art showing Flowery with a head-and-neck collar designed after Flowey's petals and a green hoodie corresponding to Flowey's stem. Flowery's final design was put together by Studio Khara staffer Touko Yatabe during the production of his introductory FMV.

The Flowey inspiration is further alluded to throughout Flowery's appearances in Chapter 5. In addition to being a golden flower in the Light World, Flowery's initial theme, "Your Dad's Best Friend", nods to the title of Flowey's motif, "Your Best Friend". The latter is itself quoted partway through Flowery's battle theme, "Flower Man". Additionally, during the battle against Flowery, the flavor text will occasionally read "Flowery emits the fragrance of hope!" and "Flowery holds onto his dream.", referencing "Hopes and Dreams", the battle theme against Spoiler:Asriel, Flowey's true form, in Undertale. One of Flowery's lines during the fight additionally muses on a version of events where he and the party didn't have to fight one another, calling it a "pacifist root," referencing the colloquial term for the Undertale route wherein the player beats the game while sparing every monster, culminating in Spoiler:the aforementioned Asriel fight.

Prior to Chapter 5's release, the "Flowery" misspelling was also alluded to in an interview with Sans and Papyrus included in one of the editions of the Undertale/Deltarune newsletter, in which Papyrus repeatedly mis-remembers Flowey's name as "Flowery."

In the September 14, 2023 edition of the Undertale/Deltarune newsletter, Toby Fox revealed that a stealth sequence was planned for Chapter 3, only to be cut partway through development due to his dissatisfaction with it, stating that "a mechanic which basically just makes you move slower isn't necessarily fun." An early version of the backstage area based around this concept is still present in the game's data, while the sneaking mechanic in the final version of Chapter 3 is heavily simplified and faster-paced.

To compensate for the original segment's removal, Fox included an MP3 file of the music that was composed for it, which would eventually be incorporated in Chapter 5 for two optional cutscenes if the player knocks on the Mike Room's door in Castle Town.

The emblem on Ralsei's robes went through two different renditions in early development. Initial concept art by Toby Fox depicts him with a fireball icon on his robe, matching a statement in an interview where he stated that "[he] was going to give a certain character a fire spell, but decided against it for the first chapter." Later pre-release concept art by Temmie Chang, meanwhile, depicts Ralsei's robes as bearing an upside-down spade symbol. Come the release of Chapter 1, and the design would eventually be changed to a heart; however, most of Ralsei's overworld sprites in Chapter 2 change this to the inverted spade design.

When speaking to Father Alvin about Asriel near the end of Chapter 2, he will mention about how Asriel used to beg for forgiveness for "dropping the lizard in the pit to jump high" in a video game. This is a nod to an exploit often used in Mario games, starting with Super Mario World, in which the player can jump off of Yoshi into a bottomless pit in order to land safely on a platform that they missed.

Within the data for Chapter 4 are unused sprites depicting Susie playing a handheld video game while hanging out with Noelle. Concept art for this was shown off in a tweet by Toby Fox in 2022, alongside artwork of two other character interactions. Notably, the Susie and Noelle scenario is the only one shown in the tweet that isn't featured in the final game.

In Spoiler:the Weird Route version of Chapter 4, one pivotal scene depicts Spoiler:Kris, possessed by the SOUL, forcing Noelle to re-don the Thorn Ring from Chapter 2 by slowly creeping towards her as she backs away in fear across a couch. In the original release of the chapter, this gives way to an animation Spoiler:of a rose blooming and then crumbling against a black screen, located in the same place as Noelle's sprite. In the Version 1.01A update, this was replaced by Spoiler:a single red pixel, possibly as a placeholder graphic, which in turn was replaced in Version 1.01C with another animation depicting cracking red glass. The original animation is outright removed from the game's files in these later updates rather than being merely disabled.

The reason for this alteration was not explained in the official changelogs, which vaguely described the changes as a "Small graphical alteration to a certain scene" and "Strengthened imagery while maintaining a scene's intended meaning". The animation changes were likely made initially in response to complaints and discussion among players who misinterpreted the scene as Spoiler:Noelle being forcibly "deflowered" (taking her virginity, in this case through rape) literally, in addition to being driven insane, and then to counter-complaints Spoiler:lamenting the reduced impact when the single red pixel was implemented.

Both earlier revisions of this sequence are referenced if the player continues the Weird Route into Chapter 5. Spoiler:The chapter's opening cutscene on the route depicts the same red pixel as the Version 1.01A rendition, after which Noelle is heard echoing her line, "thank you," which follows the Thorn Ring sequence in Chapter 4. If the player aborts the route during the lake scene, Kris' title is changed to "Shattered Rose," with the accompanying description reading "Only one knows the rose's meaning." While not the only "rose" title in Chapter 5, the specific use of the descriptor "shattered" aligns with the crumbling animation from Chapter 4's initial release.

Within the data for Chapter 2 is a placeholder graphic for Queen's arcade cabinet that depicts it with green paneling and the word "Xbox" crudely written on the marquee. Despite the fact that it was never intended to be featured in the normal course of play, the sprite was altered in the version 1.06 update to remove the "XBOX" text, presumably to avoid potential trademark issues.

In Chapter 2, when Queen challenges Kris to an arcade match, the arcade cabinet presented is so large that Kris is unable to reach the controls. While the released game conveys this solely through narration, its internal data contains an unused animation depicting Kris reaching up towards the cabinet's joysticks. It is unknown why this was removed, given that it fulfills the same function as the textual description.

In the Undertale sixth anniversary livestream, Toby Fox stated that the scene in Chapter 1 where Susie holds off from attacking Kris due to her respect for Toriel was inspired by a real encounter with a bully that he had. According to Fox, the bully knew his mother and stated that they would've beaten him up if she wasn't nice to them.

In an interview that Toby Fox conducted with the Chapter 2 development team, Taxiderby, one of the game's programmers, stated that the Spamton cherub that appears when pressing F1 during the Spamton and Spoiler:Spamton NEO fights was a spur-of-the-moment suggestion. According to them, Spamton's line "[Press F1 For] HELP" was already written, but it was originally a meaningless non-sequitur. After Fox suggested the idea of giving F1 an actual function in the fight, Taxiderby came up with the Final Fantasy-inspired Easter egg, putting it in singlehandedly after Fox gave his approval for the idea. In the same interview, they stated that they were surprised by its popularity, having made it purely as a throwaway gag.

Although Chapter 1 was given a surprise release in 2018, with Toby Fox giving no prior public indications of its existence, Deltarune had been teased in secret since at least 2015. Three months after Undertale released, the fan site DeltaRune.com announced that it would be rebranding to Dreemurr.com at Fox's request; Fox didn't tell the owners of Dreemurr.com why he didn't want them to use the domain. However, after the fan site fulfilled his request, Fox used deltarune.com to host an image called him.png, a passage of Wingdings text only readable by turning up the brightness using image editing software.

While the text initially parroted the phrase "THIS NEXT (space) EXPERIMENT (space) SEEMS (space) VERY (space) VERY (space) INTERESTING" from the unused room_gaster event in Undertale, by July 1, 2016, the text was edited to read "THREE HEROES APPEARED (space) AT WORLD'S EDGE", later being edited again by August 17 to say "THREE HEROES APPEARED (space) TO BANISH THE ANGELS HEAVEN". Both revisions quote portions of the legend that Ralsei recounts in Chapter 1, indicating that these parts of the game's backstory were already conceived by this point.

Following the release of Chapter 1, deltarune.com would be refurbished as the official website for Deltarune itself. Consequently, earlier versions of the site only survive through snapshots on the Wayback Machine, a URL archiving platform hosted by the Internet Archive.

In 2012, Toby Fox posted several Yume Nikki fan songs to his Tumblr account. One of these, "waltz of seccom masada" (named after a fan nickname for the piano-playing NPC in the spaceship area), bears strong similarities to the Deltarune track "Man" (heard in the hidden rooms where Kris can obtain an egg from the Forgotten Man, an invisible NPC behind a tree), indicating that Fox repurposed and rearranged the song for his own project years later.

A second, 8-bit rendition of the song is present in the same Tumblr post. This version would appear unaltered in Mancountry, the hidden egg room in Chapter 3.

In Volume 5 of his Famitsu column "Toby's Secret Base", creator and director Toby Fox revealed that Ralsei's name came from a period in elementary school where he and his brothers constantly experimented with RPG Maker, having long had a shared interest in game development. Fox's oldest brother spent years working on a game called New Genesis, which featured a protagonist named Ralse; Fox simply appended an "i" to the name when incorporating it into Deltarune years later.

In the same column, Fox stated that the prolonged development of New Genesis discouraged him from making games for a long time. When he finally returned to the field in his teenage years, he sought to temper his expectations from the outset by making smaller-scale titles and preemptively planning out his approach to development to avoid biting off more than he could chew.

After Queen realizes that she didn't capture Ralsei out of all of the Lightners due to not having enough cages, the sound effect that plays with the Iris Out zooming in on her face is the same sound effect used on the Iris Out on Mario when completing a level in Super Mario World.

In the background of Seam's shop, one can spot a doll designed after Yoki, a character from Deltarune designer Temmie Chang's games Escaped Chasm and Dweller's Empty Path. In Chapter 2, the Yoki doll is redesigned to match changes in Chang's own depiction of the character. The original design was initially still present in Chapter 1, but was later altered in the Version 1.08 update to match Chapter 2.