On the March 21st edition of Late Night with Conan O'Brien (1993), Director 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.
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 (Dylan Moran). 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 was cast as Mr. Goshdashtidar despite the fact that he had no experience driving a scooter. He spent several weeks taking lessons, to make sure he wouldn't run over any cast members.
The original script centered on the Los Angeles Marathon. Once a UK production team decided to make the film, the location was changed to London. Another film had already secured the rights to the London Marathon, so the filmmakers invented "The Nike River Run" to take its place.
David Walliams: the man in the bakery. The role is quite similar to "Mr. Mann" in a recurrent sketch from Little Britain (2003). He plays an annoying customer who always comes up ridiculously impossible requests.
Floella Benjamin: chancellor of the University of Exeter, and presenter of Play School on a children's show in UK, has a cameo role as Libby's mother.
David Schwimmer: In the first shot in the bus, someone is sitting six rows in front of Dennis, with his head down so you can't see his face. It's Schwimmer, watching the scene on a monitor. There was no room behind the cameraman, so he's in the shot.