A celebrity may take an uncredited role for a number of reasons.

1. They may think that the movie is truly awful, but they are contractually obligated to appear, but want to downplay their prescence as much as possible.

2. They may be doing an "independent film" and don't want their name to overshadow the movie.

3. They may be a fan of a particular series of movies and take a very small role in one of the films. A good example of this is Christian Slater who had an uncredited role in Star Trek VI: The Undiscovered Country. He even had a line to say.

Some celebrities do it for nothing more than "grins and giggles!" Lots of times you hear a star was visiting on a set and got asked to step in for something quick. Some are fans of the director or other actors, and will do it just to have the experience to work with them. Some owe a return favor to someone who helped them on their own project in the past. It's an interesting way to create a little fun buzz about a movie, for no good reason other than to have people talking about it . . .

A speaking role does not automatically meen you are in the credits. For example, Mission:Impossible included a speaking character that was NOT listed in the credits, played by Emilio Estevez. He died in the elevator shaft at the beginning of the movie (or close to it) and wasnt credited in the end of movie credits. He spoke quite a bit while he was a part of the movie.

A person has to have a speaking role in a movie to be in the credits. Although the movie "me, myself and Irene" is a good example of ALL the persons who where seen in the movie, whether thay had a speaking role or not. Sometimes however, persons who do not speak but have a role or part in a movie which stands out, can be refered to as "boy on bike" or "police officier number two" etc