RTN11
I find if someone needs to be calmed down, I do everything to reassure them of how safe flying is. I'd certainly have to eat my words if the engine quit shortly after take off with very few landing sites available, but that's the calculated risk I take.
I don't tell people it's safer than driving. I say that people say that, but really it's more like riding a motorcycle, as has been mentioned here.
I do tell passengers that if the noisy bit up front stops, that's no problem. The aeroplane doesn't stop flying and by the time we reach whatever field we're headed for, the aeroplane is going no more than 60 mph. I then point out that if you swerve off the road at 60mph in a car, it isn't going to be pretty, but if you miss the solid bits it's totally survivable. Then point out that similar applies to aeroplanes, even if at the end the plane's a bit (lot) messed up. It's not strictly comparable, but at least I have some choice of field unlike the car, and I have faith in my ability to FL with at least the people on board intact. I practice lots.
The thing that I don't talk about is those situations where a quiet donkey is the least of my problems - any scenario where I am no longer able to exert control over the aeroplane, or fire. They're the areas I won't discuss, since I don't have any good answers.
On that one, any suggestions from more experienced chaps would be appreciated...