The game was originally pitched by Telltale as a spin-off of Valve's Left 4 Dead franchise, but Valve turned down the pitch leaving Telltale to go to Robert Kirkman to successfully pitch him a game based on his Walking Dead comic book series. The same unique choice-based story-driven style of gameplay was kept by Telltale developers between the canceled Left 4 Dead project and the greenlit Walking Dead project, which the devs thought was a better fit for Kirkman's universe than Valve's franchise anyway and which would also be a style that would be reused heavily by Telltale in their later licensed games.
Contributed by PirateGoofy
According to The Walking Dead's co-creator and writer Robert Kirkman, he became interested in making a game based on the comic after playing Strong Bad's Cool Game for Attractive People. Sometime later, Telltale Games would successfully pitch their idea of a Walking Dead game to him:

"I played their Strong Bad game. I like their approach to puzzle-based storytelling. I thought they were more focused on telling a good story, and I thought they were good at engaging the player in the narrative. That's what interested me in making a Walking Dead game. They came to me with a proposal that involved decision-making and consequences rather than ammunition gathering or jumping over things; I was impressed by that. The only thing that's really special about The Walking Dead is the human characters and the narrative that they exist in. It's all about drama and loss, so I felt like doing a game with that focus, but that wasn't something that I knew was really possible. When Telltale came and told me about the way that making decisions changed the game and the way that players would be forced to choose between two bad decisions and how the survival aspect of The Walking Dead would actually be brought to the forefront – that's when I was sold on the game."
Contributed by MehDeletingLater
At the end of "A New Day", when Lee walks up to Clementine and Duck, Duck will describe a comic he read called "Super Dinosaur." This is a comic written by creator of The Walking Dead, Robert Kirkman.
Contributed by Tristan_Brown
There are unused icons in the game's data for episode one. These include a hammer, teapot, running icons, and a rag. The hammer and teapot were scrapped entirely, the running icons were cut out of episode one and put into episode two, and the rag was used in episode three instead.
Contributed by frankfrost2
Telltale hosted a pre-order contest for Mac and PC users, where the winner would appear as a survivor in-game, while four runner-ups would appear as zombies. Brie Rosenholm won the contest and appeared in "Around Every Corner" and "No Time Left". The four runner-ups appeared as zombies near the end of "No Time Left".
Contributed by SOGESNAKE
Doug once had his own subquest in early development, in which Lee had to knock the pole onto his brother while Doug attracted the walkers away with Kenny's truck. Lee had to cut the electricity before cutting the pole with a fire axe.
Contributed by Ben Paul
In chapter 4 of the third episode there's a bottle of Banang in the right cup holder. The inclusion of this drink is a running gag in most games developed by Telltale Games. It first appeared as a joke in "Sam and Max: Beyond Time and Space" as a powdered flavored drink with a funny name. Banang is possibly a parody of Tang, a fruit flavored drink sold worldwide.
Contributed by David
The character Doug is based off of Doug Tabacco, the IT specialist at Telltale Games. The appearance of Doug is based off of him as well as his behavior.
Contributed by Breakinggreens
In Episode 5: No Time Left, while in the room with the dead couple on the bed, talking to Kenny repeatedly will eventually cause Lee to walk over and face the player, and say "Randy Tudor. Good man. Damn fine man. Powerful 'stache. One of the greats."
Contributed by epicmickeyfan
In the game's files there are two unused character's.
One is named "Nick Stokes", who seems to be wearing a uniform with a badge. The badge also features a photo of the character Nick Stokes from CSI: Crime Scene Investigation.
The other is called "Jeff". Jeff is the only character who has the special walker texture.
Contributed by Ghost