Actors will kiss the other actor in a romantic scene with them, yes.
As for sex scenes, actors don’t actually have sex. They will wear tiny underwear that barely covers their genitals, and be filmed from angles that hide them. Sex scenes tend to have as few people behind the scenes as possible, but they will still have the director, camera operators, lighting and sound crew present. They aren’t actually that intimate, as the actors may have to repeat the scene over and over again if the lighting is off, if the director didn’t like the performance, or if anything doesn’t come across perfectly.
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