Can an actor sue for slander if someone writes something bad about him that is true and they can prove it?
Way back in 1735, John Peter Zenger was put on trial by the British Governor for printing accusations of corruption in his newspaper against that Governor. They charged him with sedition. His defense was that the accusations were true, and he proceeded to prove them in court. Once he did, he was acquitted, and that case became a part of the basis of our first amendment freedoms and limitations.
Let's be clear, a person can sue for anything they want to, but can they win? I would have to agree with the above post that if the allegations are and can be proven true in court, then no libel/slander exists.
Because of the internet, blogs and message boards, people do need to be more careful what rumors they post about anyone unless they are prepared to defend themselves in court.
In the United States, at least, truth is absolute defense against libel (slander is when untrue and harmful things are SAID about a person, libel is when they are WRITTEN). So if the statement is true, there can be no libel.
It is also much harder to sue for libel if you are a public figure. So if this is a famous actor, it is very hard to prove libel. The media is allowed to say much more about a public figure than about a private figure. To prove libel, a public figure has to prove that the media outlet acted with "reckless disregard for the truth."
Finally, it is not possible to sue for libel over an opinion. So if the bad thing is a review of a movie, the actor can't sue either.