Black Widow has never been so clearly rattled by anything as the prospect of going up against the Hulk. As an espionage agent, her modus operandi is to manipulate her targets into underestimating her and then revealing their plans to her in their own overconfidence—a move that works well against Russian gangsters, Asgardian Norse gods, and genius billionaire playboy philanthropists alike. In actual combat, Natasha is a master of human-level martial arts and firearms. And the Hulk terrifies her because none of her tactics work on him. The Hulk can’t be manipulated or reasoned with, and worse, his actions are completely unpredictable. Guns don’t work on him, nor hand-to-hand combat without superhuman power. No wonder she’s freaked out by him even as a seasoned agent: her only option is to run like hell. Her eventual acceptance of Hulk/Banner as a teammate is arguably the bravest thing she does in the movie.
Jimmy Fallon and Justin Timberlake in The History of Rap Part 4.