Some of the most famous scenes in the movies, never appeared in the script. They are 100% ad-libbed. So good in fact, they were kept in the final cut of the film.
One of the best scenes in Pretty Woman, where Richard Gere slams the jewelry case shut on Julia Robert’s hand when she goes in for the necklace, was a totally “in the moment” moment. Another good one…Jack Nicholson’s “Here’s Johnny” line from The Shining was also ad-libbed. Even The Godfather cat scene wasn’t in the script! That cat just happened to wander onto the set.