Nothing more scary than looking over and seeing your instructors hands are no where near the controls
One of my mates is sometimes a nervous passenger. Took him flying in Oz, with an instructor because it wasn't worth getting an Oz licence whilst on a short holiday.
So I explained: "What you need to do is look at the instructor's hands. If they're in his lap, then there isn't a problem and you're perfectly safe; if they're on the controls then there isn't a problem and you're perfectly safe."
He did understand!