Or Rutger Hauer in Blade Runner.

One of the most influential science fiction monologues was ad libbed, and it's so much better than the original script.

Original script:

"I've seen things... seen things you little people wouldn't believe. Attack ships on fire off the shoulder of Orion bright as magnesium... I rode on the back decks of a blinker and watched C-beams glitter in the dark near the Tannhäuser Gate. All those moments... they'll be gone."

Hauer's monologue:

"I've seen things you people wouldn't believe... Attack ships on fire off the shoulder of Orion... I watched C-beams glitter in the dark near the Tannhäuser Gate. All those moments will be lost in time, like tears in rain... Time to die."