Name Films where directors didnt tell actors what's happening and film a real reaction?

In the movie The Turning Point Shirley MacLaine throws a drink into the face of Ann Bankcroft but only MacLaine and the director knew it was going to happen. And in the movie Roman Holiday Gregory Peck sticks his hand in the Mouth of Truth and when he pulls it back he screams and has it stuffed up his sleeve. Audry Hepburn didn't know he was going to do it.