The movie is loosely based on the experiences writer Jon Favreau had when he first moved to Los Angeles. He had just broken up with a long term girlfriend and counted on his friends Vince Vaughn and Ron Livingston to cheer him up. The characters they play in the film are based on themselves.
The "Bear" monologue that Trent (Vince Vaughn) delivers to Mike (Jon Favreau) is almost verbatim something Vince told Jon one night at a bar. Favreau liked it a lot and incorporated it into the script.
The scene with Mike and Trent talking in the car on the side of the road was also filmed without a permit (not only could the production not afford one, it is actually impossible for any film production to acquire one to film on that particular highway). Originally, they had planned to film just an establishing shot of the two of them in the car, and a shot of them driving away, and then film the dialogue shots later. But director Doug Liman decided instead to film the entire scene on the side of the road. During filming, several police showed up, and demanded to see a permit. The assistant director held up the police by telling them that they had a permit, but it was in the office across town, several miles away. To get away with the rest of the scene being filmed, Liman had to pretend he was not filming, and didn't look in the viewfinder, and used a microphone inside of the car instead of a boom. Most of the scene was filmed like this, with the police waiting just out of shot, and the two actors and the director pretending they were in fact not shooting.
The exterior and interior of Mike Peters' (Jon Favreau's) apartment was the actual building and room in which Jon lived at the time this movie was filmed. Favreau's downstairs neighbor was actor Adam Scott.
The scene in which Trent (Vince Vaughn) angrily yells at Sue (Patrick Van Horn), after Sue insulted Mike (Jon Favreau), was written specifically at Vince's request. Vaughn wanted to show that beneath Trent's bravado and swagger, he truly cared for Mike as a friend.
Vernon Vaughn: Vince Vaughn's father plays the lucky gambler at the one hundred dollar minimum blackjack table.
Joan Favreau: Jon Favreau's grandmother is the lucky gambler at the five dollar minimum blackjack table.
Nicole LaLoggia: The film's line producer plays two roles. She plays Michelle's voice on the phone, and she appears as one of the bar patrons at the Derby (the brunette sitting to the right of Trent when Mike leaves the table).