As I recall, "Firing around a corner" gives you half cover. "Looking around a rocner" gives you three-quarters cover. If they hit the cover instead of you, then you managed to duck back down in time -- but if they hit you, then they tagged you just as you peeked out. If you have Spring Attack or Shot on the Run (or if you're fine with using a move action to take five steps out and then five steps back), you can move all the way out into the open and then back. Otherwise, you should stick with a certain level of cover as your average exposure over the course of the round -- that's what the cover rules are there for.
Does that help?
EDIT: To more direclty answer your question:
-If the hero is ducking fully behind a car and then says "Okay, next round, I wanna fire over the top," I'd just let him do that with a five-foot adjustment, moving from the middle of the car over to the hood or something. He's still kneeling -- he's just gone from full cover and no ability to fire or see his opponent to half-cover and able to fire and see his opponent clearly.
-If the villain isn't firing, but just wants to check his opponent's location, he can go from full cover to three-quarters cover with no trouble. And he should stay there all round -- I'm assuming he's doing other stuff during the round, too, like reloading his gun or barking orders into a radio.
If the only movement actions that someone does during the round are ducking back behind cover as somebody shoots and then peering out again, that's not really a move -- it's a function of the cover bonus to Defense, just like you don't charge somebody move-actions for dodging blows in combat.
