I thought of something else (while driving to Portland today) us "mere mortals" don't think about -- that I've been told now repeatedly by my cop friends... is finding cover. I think most of us have this idea that we're going to wake up in the middle of the night by someone making noises in, or attempting to get in, our houses. So half awake you get up - grab your weapon of choice and go to see what the matter (clatter?) is all about... that is when you're confronted with the element of surprise.
The trained would seek concealment/cover while they investigate the unknown..... while we'd (the untrained) just walk down the hall/stairs... and head for where we thought we heard the noise come from.
The training simulator has all these life size scenarios playing on the screen - AND they can shoot you with pellets! If you're not covered/concealed - prepare to get shot! You took cover but your knee exposed? Prepare to have a bruise there!
It's that element of surprise - coupled with the total exposure that is going to be your worst nightmare. At least that's what I've been shown.