Well of course not out of nowhere. But like when I got to know her enough to not be awkward around each other. That we like each others' company enough, and so forth.
Most Helpful Girl
It's a good idea if she really is the prettiest girl you've ever seen. If not, then it's just an empty compliment. Even if she is though, she probably wouldn't believe it and think you're brown-nosing her. It's best to give a simple and true compliment that is believable.1