Actually, it kinda does work like that. The characters going before the ambush that's triggering the initiative roll can move and do things (Ready springs to mind) before the opposition, and they have their reactions available to them.hilarious scene .:
DM: Roll initiative
Player: OK, i win
DM: What are you going to do?
Player: What do I see?
DM: Nothing
Player: What's going on?
DM: You don't know.
Player: ???
DM: what are you going to do?
Player: I know the wtf.