There's a theory on Rian Johnson's official forum that the The Brain only exists inside Brendan's mind. While Rian will neither confirm nor deny the theory, he has said it is "without a trace of irony, my favorite post on the forum."
Rian Johnson cited Spaghetti Westerns and Cowboy Bebop (1998) as influences on his visualization of the movie.
Rian Johnson wrote the original screenplay in 1997 "right out of film school." It took him six years in order to fund the project. His parents ended up funding the project for him in 2003.
Brendan's hair, personality and distinctive walk are partially inspired by Spike Spiegel, a character from the anime "Cowboy Bebop (1998)," of which director Rian Johnson is a fan.
Writer/director Rian Johnson first wrote the story as a novella, in the style of Dashiell Hammett, before turning it into a screenplay. Both the novella and the original script are available for free on his official website.