film techniques - Do the actors really go nude in the shooting of the movies?

I often wonder if actors really go nude for certain scenes in films.

I am not talking about those movies where we can see the sexual body-parts or genitals of the actors/actresses, but instead less explicit scenes. Do they also perform nude then to make the scene more realistic?

For example, I can remember at the start of the epic film Troy, Paris had sex with his lover, Helen. During the sex scene, we could not see any sexual body-parts, but the scene was shot such a way that we can understand they were nude and were having sex. In these types of scenes it seems like this type of shot is impossible because of the way their bare body parts are shown, unless they really go nude.

So what is done in these situations? Do they perform nude or not?