Ghostbusters
The falling bookshelf behind the characters wasn't scripted; it fell spontaneously during the take. The actors were startled and looked around. Bill Murray then turned to Dan Aykroyd and asked if he'd ever experienced anything like that before. The director loved the spontaneity so much that he kept the scene in the film.The Chronicles of Narnia: The Lion, the Witch and the Wardrobe
Director Andrew Adamson decided not to show young actress Georgie Henley the set built for Narnia's world. Thus, when Lucy enters the magical wardrobe, her first glimpse of the snowy forest is genuine. Her reaction is heartfelt.Titanic
In the scene where Rose searches the corridors for Jack, the water used on set was actually from the ocean. It was so cold that when Rose nearly drowns in it, it's Kate Winslet's genuine reaction.
It
Before filming the garage scene, the young actors, Finn Wolfhard, Sophia Lillis, and Jaeden Martell, hadn't seen Bill Skarsgård in full makeup and costume. That's why their terror at seeing the creepy clown is not feigned – the fear in their eyes is real.