The Great Red Eye of Disturbance!
If your actors absolutely cannot act whatsoever, or freak out once the camera’s on, make a big noise about your DP turning the camera off while you run through some off-camera rehearsals. Have the DP filming this secretly (“Just pulling focus, guys — don’t mind me!”). No matter how crappy an actor might be, they can usually be themselves with some reasonably convincing skill.
I know, it’s a bit of a trick, but I’ve seen people freeze up often enough once that red LED goes on that I’ve had to do this a few times.
It’s common enough an occurrence that I can actually get my DP to do it without saying a word, just with hand signs.
That’s been cool.
If your camera can shut the LED off, even better!