It always takes me some time to figure Quinn out. Of course, I was wrong, since the episode flat-out said that it was about acting, and who's better at acting than Quinn Fabray?

Thinking it through again... from what I can tell, Quinn's veneer of happiness gets increasingly frayed throughout the ep until she can barely manage her conversation with Joe. The sole exceptions, I think, are the skate park and the steep ramp scenes, where she seems genuinely joyful.