I think I can tell pretty well, just by observing if the actor makes me believe in the character (that the character could be real).
I haven't seen any of the vampire films but there are very, very very few actors who could top Marlon Brando's skills. popularity among schoolgirls means nothing - Orlando Bloom and Keanu Reeves were popular too, both terrible, terrible actors. or Clive Owen, who gets cast a lot nowdays, I don't think he ever convinced me to any character he played.
I dare say that Natalie Portman is a sh***y actress too and she's been in craploads of films.
from what I've noticed it's mostly supporting actors who are real good and make the celebs who appear in main roles look slightly better.
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not a bug - a feature.