Defamation cases are notoriously difficult to win. There is a distinction between an opinion and a statement of fact, for one thing. I can sit in my own home and say "I think (a famous actor) is a drunken idiot" all day long. I haven't said it or wrote it publicly, and the actor hasn't lost any work because of my opinion. If I took out an ad in Variety magazine that read "Don't hire (a famous actor) because he's a raging alcoholic", then I could be sued for defamation. I have damaged the man's reputation in a very public way.
The other problem is that the accusation must be false. Embarrassment is not the same as defamation. The famous actor may not want his problems with alcohol aired publicly, but it's still true. It's still not defamation of character. In order to win a defamation lawsuit, the accusation would have to be proven false and there would have to be evidence that others believed it and took action because of it.