6) Thor: Ragnarok (2017)
In Thor: Ragnarok, Loki, played by Tom Hiddleston, has to contend with the Hulk. That has been something that occurred before, and Hulk had famously been known to slam around Loki like a rag doll when the duo faced each other in The Avengers. Hiddleston was not ready for the improv moment when Hulk suddenly grabbed Loki while fighting one in a gladiator battle. The genuine look of terror and the line, "I have to get off this planet," were very real.
Hiddleston's impulsive reaction did just what was required for Loki's character arc by infusing the comedic and dramatic aspects into the scene. Director Taika Waititi gave the freedom to the actors to improvise, and this moment of real fear gave them one of the most hilarious callbacks in the Marvel Cinematic Universe.