No, no, no!!! The rebel spy is a GOOD guy!!! It means you're working for the Resistance, and Darth & his minions are after you!!! You're the HERO!!!
I've actually been chosen just once, in the hundreds of times I've ridden this ride. I was riding solo - so, so much for being part of a large group. I was in the back row, in the corner - so, so much for sitting in the middle. I WAS looking serious, and wasn't smiling, though, as I have somewhat of a RBF (if you get my drift.) I think it's just completely random.
I think mine is that I always had my sunglasses off and was probably the first person buckled in and ready to go! I'm always smiling that "I'm nervous, I love this ride, until I think I might throw up" smile, if you know what that is.