I don’t think that I ever saw an actor who I thought did a “bad job”, literally the only bad acting I saw was that in parody movies, like Extreme movie and so on, but I loved even movies like that, since that’s the whole point of the movie, and yet, they have very low ratings…
If you took a random CSI or X files actor who appears in only one episode and not even as a main antagonist or something and if you took the main actors from the biggest AAA movies, I would see absolutely no difference in “believability” of their acts, in fact I’m probably going to prefer a no-name actor, than to see Adam Sandler, Will Smith, Sean Scott (Stifler), etc, since I am always going to associate their faces with their previous roles and genres.
I never disliked a movie because of the level of believability, I can dislike the storyline, the setting and environment, the pace (for example a boring movie that seems to never end), a genre,etc, I may even dislike an actor because I just dislike how he looks, he has an annoying role and so on, but none of those are related to believability, so I can’t understand people that give low ratings to movies due to that.