Why Did Will Ferrell Write More Cowbell for Walken?

Will Ferrell got the idea listening to Blue Oyster Cult on the radio and hearing that faint cowbell, wondering whether the cowbell player had any friends and if that was the only song he got to play on. He wrote it as a sketch, then rewrote it for Christopher Walken, because only Walken could get a laugh out of "I have a fever, and the only prescription is more cowbell." The stiffer Walken played it, reading off the cue cards, the funnier it got.