On the March 21st edition of Late Night with Conan O'Brien (1993), David Schwimmer discussed the scene between Hank Azaria and Simon Pegg that takes place in the locker room. Generally, if males have to be nude for a scene, they will use a nude (flesh)-colored pouch to cover themselves. However, for some reason, the pouch wouldn't stay put and Azaria ultimately decided to forgo wearing it - but told Schwimmer not to say anything to Pegg. The reaction of shock we see from Simon Pegg when Azaria drops his towel is genuine.
David Walliams has a cameo as the Man in Bakery. This role is quite similar to his part as "Mr Mann" in a recurrent sketch from Little Britain (2003). In this sketch he plays an annoying customer who always comes up with the most ridiculously impossible requests.
Throughout the film there are several different cartoon versions of Dennis that appear as graffiti. Each represents Dennis' state of mind at that point in the movie. The easiest one to see is right after Dennis has the fight with Gordon. He runs around the corner and leans against a wall then slowly sinks to the ground. Behind him on the wall is a cartoon Dennis hanging from a tree representing his lowest ebb in the movie.
Harish Patel, who was cast as Mr. Goshdashtidar, was cast despite the fact that he didn't have any experience driving a scooter, he spent weeks taking lessons to make sure he wouldn't run over any of the other actors.
The original script centered on the running of the Los Angeles Marathon. Once a UK production team decided to make the film, the location was changed to London. Because another film had already secured the rights to the London Marathon, the filmmakers invented "The Nike River Run" to take its place.
David Schwimmer: In the first shot in the bus there is a person sitting six rows in front of Dennis with his head down so you can't see his face. It's the director David Schwimmer watching the scene on his monitor. He is in the shot since there was no room behind the cameraman in the tight confines of the bus.
The trivia item below may give away important plot points.
When Dennis and Whit are lined up getting ready for the race there are two people in costumes standing behind them. The person in the hare costume is behind Whit, while the tortoise is behind Dennis, the character placement predicting the outcome.