Brief your pax on what you want upfront......no touchie no feelie airplane until you've pre-flighted and ready to assist. I learned the hard way when I was pre-flighting a 172 and had both doors open after showing the interior. The aircraft was parked on a slight upgrade with the tail low. Much to my surprise the guy I was going to take up (non pilot) was on the opposite side of the plane when he decided to just hop in the back seat for fun.........I just about passed out on the spot.......visualizing the plane dumping on the tail........by sheer luck and the fact the guy was about 165lbs nothing happened..........will not let that happen again and hopefully nobody else will either.......watch the non pilots like a hawk.................