Twizzler will just nip a couple times, when he has to go. Oliver, on the other hand, will headbutt whatever part of you is closest to him. And if you still don't get the signal, then he'll climb up your chest and bite your lip, then chitter until you put him back.
I think they're a lot smarter than people realize. All is quiet (aside from the occasional tiff) while I sleep...but the minute I wake up and pull up the room-darkening shade Twizzler and Oliver are both standing up at the side of the cage waiting for me to fill their food dishes. If I wait longer than 10 seconds, Twizzler will scream until I feed them! Guess this goes to show who owns who in this house, heh. lol