I usually brief the following things (of course in the event they have flown with me before, the briefing is shorter):
- how to get into and out of the airplane without damaging it (and without my assistance in case of emergency)
- use of seatbelt
- fire on start procedures (they evacuate while I deal with the fire)
- location of fire extinguisher
- position during takeoff (especially for the person sitting on the front right-hand seat) - hands on knees and feet off the pedals
- intercom communications (no talk from before start until I say it's OK - usually when out of runway heading)
- air sickness (I always have at least one sickness bag for each pax)
I always comfort them by explaining that possibility of an emergency in a light aircraft is very remote, but nontheless I've been trained to deal with them and have reviewed the procedures the day before (something I do just to keep them in memory). If this is their first flight with a light aircraft, I explain that there will be different forces, which could be exaggerated, but I tell them there is absolutely nothing to be afraid of and that I will try to make flight as comfort as possible. For the passengers slightly scared of heights I try to explain that higher is safer, but I usually fail at this
The most difficult thing during briefing is to keep pax from running away when they see 35-year old C172 that we are about to fly in
