When Eddie Dombrowski is shown during the Dog Ending credits Mira starts growling. This is most likely in reference to Eddie having killed a dog before, it's speculated that Mira could even be the dog Eddie killed.
Masahiro Ito, who designed the monsters for the game, said that he was inspired to create the Lying Figure when he saw a coworker walking toward him from a distance. The coworker was wearing a sweatshirt with a hood and had his hands in his pockets. He was listening to his Walkman and was walking in a "cool way."
Masahiro Ito's initial concept for the monster that would become Pyramid Head did not include the iconic helmet. The helmet was added to suggest pain.
There is persistent butterfly and moth symbolism surrounding Maria, who has a butterfly tattoo on her abdomen. Spoiler:Butterflies are seen as a symbol of rebirth in some cultures. Since she is supposed to be the reborn image of Mary, she could be seen as a butterfly. Likewise, moths are seen as a symbol of death, and Maria transforms into a swarm of black moths for the final boss. This parallels how James killed Mary.
Guy Cihi, who played James Sunderland, was only present at the auditions because he was escorting his daughter there. After reading a script in the break room, he asked if he could read for the part of James.
There is a hidden ending which reveals that a dog had been controlling all of the events in the game, followed by the credits, accompanied with circus music and random pictures.