A screenplay is an integral part of the filmmaking process, but we’ve learned enough by now to know that the finished product is rarely a 1:1 recreation of a movie’s script. Lines are changed, scenes are cut, themes are tinkered with, characters are added or removed. A little change is normal. Sometimes the biggest changes happen in the moment—  and end up being the best parts of the movie.

We love a good ad-lib, and plenty of movies are full of them. Some, like the studio comedies of old (i.e. the ancient days of the mid-2000s), were basically built on the knowledge that the actors would riff off each other on shoot day, so a writer could basically pencil in “Paul Rudd says something funny” in lieu of a line. What happens, though, when the ad-libs themselves become the best part of the movie? This doesn’t just happen in comedies—dramas, space operas, and action movies have them, too.

It can be hard to believe that a lot of the iconic lines we love weren’t sweated over by a team of writers hunched over a typewriter, but were instead tossed off in the moment as an actor was really feeling their character. It’s a testament to the actors’ ability to know what fits best in a scene, and to the directors for knowing when to let serendipity take the reins. The world where none of these lines exist because everyone stuck to the script is a close one, so let’s be glad the people behind these great movies were open to a little spontaneous inspiration.

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