Sometimes the best way to get a screen worthy reaction from an actor is to actually put them through an ordeal, physical or otherwise, so those screams and cries of pain come from a very real place.
Want a child actor to have the perfect emotional reaction to the fictional death of his character's beloved bird? Tell the kid his avian co-star died just before filming:
Cracked compiled 5 Amazing Performances By Actors Who Weren't Acting a while back, which is the third in a series, which is full of fun moments when actors really earned that on-screen credit. (Contains NSFW language)
My personal fav- the trials endured by Arnold Schwarzenegger while filming Conan The Barbarian, which was one "cut!" away from becoming a snuff flick: