Max Pomeranc was chosen because he is, in real life, a chess player (or was at the time of this movie). The producers wanted someone who would be at ease and "correctly" playing chess. None of this movie's other stars played chess in the beginning, but eventually Joe Mantegna learned.
Many of the characters who were famous chess players were actually played by themselves (Kamran Shirazi, Joel Benjamin, Roman Dzindzichashvili). The one exception is Asa Hoffman. The real Asa Hoffman did not like the way he was depicted in the script (he is shown as being neurotic, when the book describes him as being quite self-aware), and refused to cooperate, so he was played by Austin Pendleton.
The character of Jonathan Poe (Josh's young rival) was based on real-life young chess prodigy Jeff Sarwer. In the National Primary Championship, on which the climax of this movie was based, Josh and Jeff tied for first place, after which Josh won on tie-breaks. While Sarwer would go on to win the World Championship Under 10, he soon disappeared with his sister and father. The family was known for living a travelling lifestyle with no permanent address. Jeff Sarwer, upon seeing the film, had no opinion on his character being portrayed as Josh's single-minded rival, stating, "At the end of the day it was a Hollywood film, a work of fiction, and it helped popularize chess more so that's always a good thing. But I have a lot of distance to the actual book and film. The way I was portrayed was nothing at all like how I was in real life, so what's the point in comparing myself to it?"
Near the beginning of the movie, when Josh's mom picks him up from school, the shorter, brown-haired woman she is seen talking with briefly is the mother of the real Josh Waitzkin.
The real Bruce Pandolfini is very different from the character Sir Ben Kingsley plays. Pandolfini is actually a Brooklynite with a mustache, glasses, and curly brown hair, and he is reported to have a very friendly, soft-spoken demeanor. In fact, Pandolfini can be seen in a cameo role in the movie. He is a kibitzer when Kingsley-as-Pandolfini goes to see Josh in Washington Square. Pandolfini tells "Pandolfini" that Josh is a "young Fischer". Josh Waitzkin is quick to point out that the portrayal in this movie is fiction, and should not be confused with reality.