Why do some actors go uncredited in films or TV roles?

Is it because they want the paycheck but don't want to be associated with the film, or is it a SAG thing?

• Do you mean unknown actors or like when a star isn't credited? Sometimes stars don't take credit because they are unhappy with the film and don't want to be a part of it.....or sometimes they are playing a smaller role than ususal (think Bill Murray in Tootsie) and the non-credit makes it look like they are doing a favor to the film and not just reduced to a small role.

• I mean known actors. Sean Penn had a small role in the late '80s (or maybe early '90s) movie "Cool Blue", and wasn't credited. Ricki Lake was in the "Buffy The Vampire Slayer" movie and is uncredited.

• Ethan Hawke had his role cut from the new Total Recall but is still in the credits.

• It varies. Don Cheadle was uncredited in Oceans Eleven because of a dispute over place in the credits. He also went uncredited in Rush Hour because he did it as a favor and didn't feel necessary to be listed.

• Wasn't Brad Pitt uncredited for his funny cameo as a pothead in True Romance?