Pretty Woman (1990)
Director – Garry Marshall
In what became one of the most famous scenes from the film, Edward Lewis (Richard Gere) presents call girl Vivian Ward (Julia Roberts) with a gorgeous and rather expensive diamond necklace. As Roberts reaches out to touch the precious jewels, Gere – in an unscripted playful moment – quickly snaps the box shut genuinely surprising her.
Her laugh was so honest, and the scene so good, that Marshall decided to leave it in the film as is.