While in spirit and tone, the Max Payne film is very similar to the game, many details are changed. Obviously, numerous things are cut because the film runs at about 2 hours, while the play time of the game is much longer
The character Jason Colvin (Chris O'Donnell) is a completely new character added for the film. Max's inner-monologue and "comic book" style narration are almost completely cut, except for minor bits in the beginning and end of the film. Jack Lupino's origin is revealed, as he is portrayed as a "test subject gone mad" in the film. He was given Valkyr as a sort of "super serum" for soldiers in the War on Terror, and, like almost all the other test subjects, eventually went insane. In the game, Jack Lupino dies in the first section, whereas in the film he dies in the final act. Also, in the game, Max kills him, while in the film, he is killed by B.B. Hensley. Nicole Horne does not die in the film.
Unlike in the game, Mona Sax is not hired to kill Max by Nicole Horne. Mona is not wounded or presumably killed as she was in the game. However, a slight reference is made to that game scene in the film, as she is last seen in an elevator. Jim Bravura is no longer the Chief of Police, but instead an Internal Affairs detective, and is turned from an aging police veteran to a young, upstart, and wise cracking detective. All of the dream sequences from the game are cut, and had they not been cut, the film would have most definitely received an "R" rating. Valkyr is explained much more in the film, and displayed on film as a massive hallucinogenic drug, still created by Nicole Horne's company, Aesir. In the film, however, it was a failed "serum" used on soldiers in the War on Terror to make them "fearless". The drug made the test subjects so hungry for more and insane that Aesir was forced to shut down the project, not before Jack Lupino had escaped to the streets. In the film, the hallucinations of the users are mainly demonic angels which "influence" the user's choices, and the drug also turns violent images into peaceful ones, and peaceful images into violent ones. The best way to describe the effects of Valkyr would be that it turns the user into a sort of paranoid schizophrenic.
Also, in the game, Max Payne killed all three men who killed his wife and daughter when he encountered them when he came home from work. In the movie, one of them got away. It is later revealed that B.B. is the third killer. Also, B.B. is a young guy in the game while in the movie, he is an older man.