Play the character, of course. How many Hollywood bombshells are out there? I'd say for a good portion of them, they knew the movie wasn't going to be great, but there they are, on the screen. And then they're obligated to go promote this disaster of a film, smile and tell the interviewer how people should watch it. Sometimes it's better acting than what's on the actual screen. I think Pattinson was in a unique position because he was contracted for the three movies (am I right? three movies? I can't be arsed to look it up). He was a draw with the fanbase, and there was no way to replace him. I think it's rather ballsy to speak your mind like that, instead of pretend. Good on him.
You know what 50 Shades of Grey is about, don't you?