Interesting fact:

Screenwriter Joss Whedon found Donald Sutherland extremely difficult during filming of "Buffy the Vampire Slayer." Sutherland reportedly re-wrote most of his dialogue and scenes, often making them incomprehensible in Whedon’s view. Whedon is on record for praising Sutherland’s abilities as an actor, but he has called his behavior “rude” and Sutherland himself a four-letter word.