Say you're sitting on the train, and say a guy you half-know (because he's in the same class as you, has spoken to you once 3 years ago) casually ends up sitting just in front of you. Say you have a crush on this guy, but you know he has a girlfriend, thus you (probably? I don't know about this) don't really think about the possibility of hooking up anyway.
How would you feel? How much embarassment on a scale from 1 to 10? How would you prefer the guy to act? Talk with you about casual stuff? Would that make you more comfortable, or less?
P.S. you've never explicitly told the guy that you like him, but you've looked at him/followed him/etc. in such a way that he kind of has a clue about your feelings.