Andy Garcia sat in as sidekick. Sheryl Crow led the band.
And where was Kimmel? Oh, he was there, all right: gagged and tied on the back of the set, as Damon took over.
"Just for starters," Damon told the ecstatic in-studio crowd, "let me ask you guys this: As an audience, is it weird to see a person with actual talent host this show?"
Admitting he'd been waiting a long time to do this, Damon – before turning his monologue over to Robin Williams – struck a personal note and said, "This is like the time I lost my virginity, except this is going to last way longer than one second."
Damon, tongue planted firmly in cheek for this all-star publicity stunt – earlier this month, Kimmel's show was moved into direct competition with CBS's David Letterman and NBC's Jay Leno – explained the basis for the Kimmel-Damon feud. The comic always wanted to be an actor, claimed Damon.
"I beat him out for every role he ever truly wanted," said the Bourne series star, now starring with Krasinski in Promised Land. "Jimmy has auditioned for every movie I've ever been in, every single one of them. How many did he get? None. So, he hates me."