Re. The Blade Runner quote - and yes it is amazing - Rutger Hauer improvised it on the spot, and the cameras rolled and collected an amazingly powerful piece of method acting.
Similar was Robert Shaw's speech in Jaws about the men being eaten by sharks. It seems he turned up drunk on set one day to shoot this scene, and was sent home. He returned the following day and reeled of this piece of improv, which naturally made the final cut.
My own favourite is from The Craft, a film about teenage witches in modern America. As the trio get off the bus to cast their first spell, the bus driver tells them "Watch out for the weirdoes in the park ...", to which their leader replies - "Mister, we ARE the weirdoes in the park."
Wonderful.