" When the film has two, or even three, big stars, they all may want to do this. Thus the multiplicity of producer credits. 

When it comes to writers, though, the situation is different and more complicated. The Writers @Guild of @America allows only three writing credits on a feature film, although teams of two are credited as one, separated on the credits by an ampersand ("you & I"). However, if each person works independently on the script (the most common system), the two are separated by an "and" and credited as "you and I."  But wait! You wrote the story on which the script is based, so you get "story by" credit, and your credit for the screenplay precedes mine, even if I wrote most of the script, except that if my script made substantial changes to your story, I'll get first "screenplay by" credit. If more than two of us worked on the screenplay, the credits will probably read something like "screenplay by you & I and he and she." You and I worked as a team, but he and she worked separately. It actually does have a certain logic to it, when you think about it. After all, the movie "The Flintstones" had, by various counts, at least 35, and possibly as many as 60, writers who worked on the script. Somehow the system found a way to not list most of them, and for that we can be grateful. 

The @Directors @Guild of America permits a film to list only one director, even when it's generally known that two or more worked on it. Except in very rare cases (a death in mid-production, and it had better be in the very middle of mid-production) there is only one directing credit. This is very good for a director's ego, certainly for the one who gets the credit, but also for the one who doesn't -- particularly if he or she had started production and then been removed by the producer at the insistence of the star or the studio. The public won't know that the removed one either screwed up or incurred the wrath of those more powerful. In either case, the removed director will live to direct another day. More rarely, a director will take the initiative and leave a production because of conflicts that cannot be resolved (usually attributed to "creative differences"), again usually with the studio or the star. All of this will most likely take place either before or during the first few days of shooting. 

But wait, there's more! You may at some point have noticed the name @George @Spelvin or @Georgina Spelvin or G. Spelvin or the like among the acting credits on a film. That's traditionally the alias used by actors who for one reason or another do not want to be credited with their own names. (The original Georgina Spelvin was the star of a famous porn film). There can be lots of reasons for using the alias, ranging from unhappiness with the way the production turned out to conflicts with the director or producer, or simply as an in-joke. In the same way, directors have sometimes used "@Alan @Smithee" as their alias when they didn't want to be credited under their real name. In 1997 screenwriter @Joe @Eszterhas (best known for writing "Basic Instinct") even had the bright/stupid idea of making a movie called "An Alan Smithee Film." Not only did it bomb at the box office, but the real director -- @Arthur @Hiller -- took his name off the credits, so it truly was an "Alan Smithee" film. 

Now to the below-the-line credits. When the movie fades to black, a new set of names begins to scroll up the screen. Sometimes the first credits we see will go to the production crew, the people who worked on the shooting, and sometimes they will be for the cast, often in order of prominence in the film, though sometimes in order of appearance or in alphabetical order. The production crew credits will be where all the "funny" titles start coming up, and here's what they do: The "gaffer" is the chief electrician. He (or sometimes she) works for the director of photography, setting up all the lights as they are needed for shooting, directing a crew of other electricians, preparing the basic lighting for the next scenes to be shot, ordering all lighting equipment and supplies, and so forth.