That's a hard one, as I love 2 of the roles I've had (and they happen to be the ones I'm currently proficient in). I love character attendant (I have fun working with the characters, and being able to work in all 4 parks). But I also love working at GMR (which is where I'm currently at). So it's great being able to work at both.
For the second question, I think the interviewer does pay attention to the roles you want (and try to place you in one of your choices). That being said, I have always told people to only pick roles you feel comfortable doing (for that reason).
And finally- no unfortunately. Once you get a role, it's really hard to change (99% of the time, the only reason to switch is a medical reason).