On a March 21st appearance on _"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.
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.