While I was watching American Hustle I was surprised to see Robert De Niro appear in it since he was not listed as a cast member anywhere. One occasionally sees well-known actors appear in films without any billing and I wondered about why that was the case since their presence would usually make the film more appealing.
In looking into it, it turns out that there are many reasons for doing this. One is that big stars usually have to be paid a large amount of money that is computed based on their previous films in which they got billing. If they like a small part in a film or the filmmaker is a friend for whom they are doing a favor and are willing to do it for a much lower fee or nothing at all, then they might agree to not have billing so that it does not hurt their their price for future films. Since they know they will be recognized anyway, it does not really matter to them.
Another reason is how they will be billed, which is a big deal in film marketing. If you are the only actor above the title, that means you are big enough to carry the film alone. Sharing it with others (the fewer the better) above the title is the next best. Then comes the ‘also starring X as Y’ at the end of the acting credits. If none of those are available, then not being billed at all is an option.
At other times, being uncredited is done to surprise the audience, which can also be achieved when the credits are at the end. The biggest surprise I had was in the comedy Tropic Thunder where I had the vague feeling that a prominent character looked vaguely familiar but I could not quite place him. I was really surprised to find out at the end who it was because it was so far against type.
Here is a clip in which the character appears with Matthew McConaughey. You are free to guess who it is or not watch it if you haven’t seen the film. Note that it contains strong language.