If you're going to try and figure out what your fellow man might do in a crisis don't bother. That's like trying to predict the weather in Florida. Once you think you've found the pattern, it just up and changes on you yet again. People react differently to danger because they are different. It's not complicated. I can't tell you how many times I've seen huge men fall to pieces when the zombies start their attack, and I've seen guys who are a hundred pounds soaking wet fight like the devil was about to bite their ass. You just never know. But as I've seen a lot of people I've come to trust my instincts. That big guy with all the tactical gear might not hold up as well as the guy who just managed to show up yesterday carrying nothing but a hammer.
Don't be afraid to ask questions of newcomers. If your warning bells go off, avoid. They will get you killed and then they'll steal your stuff. Bet on it.