The 53-year-old actor showed off his recent weight loss, boasting, "None of my clothes fit me".
"If you put your hands in your pockets, your pants come down," said Alec, demonstrating his point for the audience.
During the interview, David brought up Alec's run-in with the paparazzi on Tuesday. The 30 Rock star was going to get a marriage license with his fiancé Hilaria Thomas when he was accosted by a photographer, who is alleging that Alec punched him in the face.
"If I had punched him, I would be in jail right now, rightfully so," Alec said.
"I kind of pushed him out of the way because he almost hit me in the face with the camera."
Alec's run-in was published on the front page of The Daily News, with a photograph showing the altercation.
"The picture, without knowing the true story, tells a tale that's not often true," said David, showing the audience the photo of Alec pushing the shutterbug.
"You can see that you [Alec] are lunging for the camera. You got both hands going after the camera; you're not punching this guy."
Alec's own response to the incident was to tweet the phone number of Daily News editor, Colin Myler:
"Call Colin Myler, 212.210.XXXX, and tell him what a great job he's doing bringing down the once great Daily News."