I guess I have understood why actresses need to be beautiful, or better say fascinating, have that something... Imagine indeed you're a man playing a scene on top of something with a romantic view behind: you invited your girlfriend to spend the week-end in a nice place abroad to ask her to finally marry you; imagine then that she refuses, putting forward reasons like she hasn't yet got a permanent position and would prefer to reach financial stability before doing such a big step, or things like that... Or anyway, let's say she kindly refuses your proposal for some not completely unreasonable reason, almost as she would like to accept it, but she really cannot: in this kind of scene, at this moment you have to show some kind either of comprehension or of disappointment; imagine the director goes for the second one: you will try to mimic your disappointment by maybe starting moving your right arm to reach her head and give her a caress, but then slowly stopping it until you will leave it suddenly fall by your side and look downwards. In a scene like this, the feelings you will transmit by sketching faint movements of your face will be fundamental, but how on Earth are you supposed to show sorrow, regret, if looking at the actress in front of you is not raising any emotion in you?
This means, in the end, that if you want to pretend, you have yourself to believe in it at least a little bit. Or am I a bleak man who doesn't understand anything of the art of acting neither?