Mark Wahlberg and China Chow ended up in a 4-year relationship after filming this movie.
Keiko was originally supposed to be a high school student, intentionally as an homage to Asian schoolgirls in animé cartoons. However, certain powers that be found it to be too risqué, and she is now a college student in the film.
In the script, Melvin had more of a backstory, to explain why he was afraid of people disliking him, and why both his parents were dead. However, due to time and budgetary restrictions, this element was cut before shooting began.
In the original script, the Video Store Kid was only a voice heard over the phone. He was never seen until the end of the film. However, when Danny Smith was hired, it was decided to insert his scenes into the film, and introduce him physically earlier.
The scene with the car chase through the forest was filmed in a public park in Pickering, Ontario, a small suburb near Toronto. Filming took about two weeks, during which, the public was not allowed near the park. Residents of the town were understandably upset.