From what i've seen, some hold there guns tighter, some looser. They put the butt of the gun in different places on the shoulder, and hold there hand on different places on the forearm. The other day, my neighbor was holding so far up the forend he was on the bbl.!!

You already mentioned flinching....

I've even see many folks holding there guns slanted instead of straight up.

Most have never had formal gun training, and have learned a huge amount of "bad" habits...

Drilling Man