If you are a trusting individual, I recommend putting it in full auto or metering it yourself before giving it to someone else to shoot. Make sure the shutter speed is high enough, and make sure it's stopped down more than 2.8 because they will probably not focus carefully.
I've had too many shots that were focused over our heads on the background. I usually compose the shot, focus it, set AF to manual and give it to them. This would only be in situations that I don't have a tripod or something to rest it on. Although I did this once and the guy started backing up, and he was so shocked when I started yelling... NOOOO!
Excellent advice. Let's add telling the guy that you like to be shot from this spot.