The actor was on hand to promote his role in Gone Girl as well as discuss his memoir Choose Your Own Autobiography, but talk quickly turned to his own Italian wedding, which took place earlier this month at a castle near Perugia. ("Take that, George Clooney!" Harris said kiddingly.)
The ceremony featured a performance by Elton John, who also introduced Harris and David Burtka's first dance – soundtracked to Kelly Clarkson's "A Moment Like This," it featured an expertly timed fireworks display.
Harris's interview got a little more risqué when he started discussing his Tony-winning run in Broadway's Hedwig and the Angry Inch.
When Letterman noted that Harris's performance including spitting water on an audience member, the actor, 41, added that there was a fair amount of "dry-humping," as well. Letterman asked if "the Tony people" knew about that, causing Harris to quip, "That's how I won the award, Dave."
Conversation also touched upon Harris's Gone Girl sex scene, at which point Letterman jotted down a note: "Shagging, dry-humping, bare ass: It's the hat trick. You got 'em all in. Never happened before."
The David Fincher film, which also stars Ben Affleck and Rosamund Pike, opens Friday, while Harris's memoir hits shelves Oct. 14.