I totally agree with TheFacialPhilosopher. Enthusiasm is very important. I DO love it when they really love it. Sometimes in American porn, it's difficult to say whether it's played or not. I would not say in general that everything is played.

When there's enough enthusiasm involved I can even like a scene with an actress I usually don't care about, for example Sasha Knox in American Bukkake 29 and American Gokkun 11.

Sometimes I can like a guy cumming on a girl's face when she doesn't like it. The most boring scenes are those where the girl seems to be completely indifferent... but I am far off topic here....