My main question is this: do you wait to get a glimpse of the person who comes through the door first, or do you simply pull the trigger at any movement behind/coming through the door?
As peetza said - a cardinal rule: Be SURE of your target before shooting. Although no one except me and my wife is supposed to be in our house, I'd still look first, along with shouting, "STOP OR I'LL SHOOT!" Any further movement (except running backwards out of the house) gets the trigger pulled.
Remember, the only good gunfight is where no shots are fired.
As for lighting... I'd be in the dark with the top of the stairs lit up. Kneeling, one eye and both barrels around the door frame.