On the 737, A320 and most if not all transport jets, the exit doors are plug doors. They're larger than the door opening, and generally open inwards to some extent, before being mechanically positioned so that they can fit through the smaller opening. Thus, with the pressure differential of anything from 6 psi or more, the pressure forcing a plug exit on its frame is many hundreds of pounds (6+ psi x area of door), and prevents opening whilst the aircraft is pressurised.
Sorry if i'm offtopic here but the doors on the 320 are NOT plug type, they open UP and out, not in.
And trying to be ontopic ... i don't think you can open a door in the 320 in flight, the door will have to slide up and with some pressure i think it may not be the easiest thing to do (however i wouldn't try this)
