Why do famous actors and actresses occasionally appear in films without credit? Some examples off the top of my head are: Jason Robards in “Bright Lights, Big City”, Robin Williams in “The Adventures of Baron Munchausen”, Gary Oldman in “Hannibal”, the voices of Bruce Willis & Demi Moore in “Beavis and Butthead Do America”, the voice of Kathleen Turner in “Who Framed Roger Rabbit?” and Christian Slater in “Star Trek VI: The Undiscovered Country”.
One possible explanation is that they wanted to be in the movie but did not have the image the producers/directors wanted to widely associtate with the advertising.
Or maybe they’re just bored 
I have heard sometimes that big-name stars appear uncredited in small-budget movies because there is some rule (possibly related to SAG or some other union) about names appearing in credits and pay, but my recollections of this start getting fuzzy right about there.
It’s not a rule, but if an actor takes a small part, sometimes it’s because of the belief it can hurt his negotiating a fee for another film (“Why should I pay you a star’s fee? You played a bit part in your last movie?”).
Sometimes it’s just because the actor doesn’t want credit, like James Earl Jones as the voice of Darth Vader. He felt he didn’t actually play the part, so he shouldn’t be credited. I think it was the same with Kathleen Turner and Amy Irving as Jessica Rabbit (Amy did the singing voice – evidently, Kathleen can’t sing).
If an actor does a cameo (Mel Gibson in “Father’s Day” or “Fairy Tale”), they don’t get a credit primarily because a cameo is supposed to be a surprise and putting the name there would ruin things. Other times, an actor appears as a walk on just for fun (Slater in STIV, Joseph Cotton in “Touch of Evil,” Walter Huston in “The Maltese Falcon”).
Now thats just plain silly. When you think of Darth Vader, do people remember James Earl Jones or do they remember David Prowse (www.imdb.com)?
According to IMDB, Irving was credited with Jessica Rabbit’s singing voice and a Betsy Brantley was credited as “Jessica’s performance model”, but Turner went uncredited oddly enough.
<NITPICK> STVI </NITPICK> 
In the 50,s several Hollywood directors , who had been blacklisted , came to the UK and directed films .Sometimes they went uncredited and other times used false names.These films sometimes found their way back to the USA.
Ah, now this I have heard about. Is this the origin of the whole “Alan Smithee” convention, or did that just come about as a means for a director to disassociate from a bad project, like David Lynch did with Dune?
Walter Matthau was in Earthquake but it was a small, crappy role. He was unhappy with it and in the original credits his part was credited to his name at birth, Walter Matowski or something similar.
Haj
Christian Slater got into the Trek movie because his mother was the casting director.
Bill Murray did not get screen credit initially for Tootsie becasue he didn’t want people to think he was the star.
The reason for their behavior was because of the blacklisting during the anti-communist witch hunts. The only way they could get work was to take the action they did .If their true names had appeared on the credits the films would not have been accepted in the USA.Another example – Kenneth Branagh was uncredited in “Swing Kids.” Not sure of the reason, though.
Allow me to point out that David Lynch was rightly credited with the theatrical and DVD/video release of Dune. The issue at hand was with the television version of Dune.
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*Originally posted by msmith537 *
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It’s not silly at all. I remember David Prowse. He made quite an impression upon me as Julian in Kubrick’s “A Clockwork Orange”. Man sets a long shadow. Jones was just the mouthpiece.
" You’ve disappointed me for the last time " 
Cartooniverse
re Darth Vader.
Jones claimed that was the reason he didn’t take credit, but it could easily have been because he didn’t want to be associated with the movie if it flopped, or because Lucas didn’t want the name to get out.
re “Alan Smithee”
The name only became popular in the 1960s (though there is one example from 1955), too late for the red scare.
It wasn’t directors who were blacklisted as much as it was writers (well, Abraham Polonski was both, but didn’t direct again until the 60s). Once an actor was blacklisted (say, John Garfield), he just couldn’t get work. Writers could use a front (and I must stress this due to other issues – the real name of the writer was revealed during the writer’s lifetime).
Others – technically, Boris Karloff was not credited as the monster in “Frankenstein” (at least in the opening credits, when it counted). The Monster was played by “???”
Robin Williams was uncredited for his part in “Dead Again.”
Marlene Dietrich wasn’t credited for “Touch of Evil.” It possible that the part didn’t fit her glamourous image.
Lending credence to that theory is the fact that JEJ is credited in the special edition.
I don’t remember the exact details, but Robin Williams was uncredited in “Munchausen” because of contractual obligations to another film he was making at the same time.
Bruce Willis and Whoopi Goldberg are uncredited in **National Lampoon’s Loaded Weapon 1, IIRC.
I know that Spike Milligan (of Goons fame) was included in the “sandel” scene in Monty Python’s “Life of Brian” because he was literally walking by the set one day and bumped into one of the Pythons, got chatting and had nothing better to do with his day.
I don’t know if he got credited or not…
Gp