Mary-Kate Olsen, 22, is reportedly making a guest appearance on the ABC hit show Samantha Who?. The filming is taking place this week for fall airing. Her film The Wackness is also set to open in select theaters on July 3rd.

The reclusive millionaire reportedly has good comedy instincts, no doubt the result of her many years as a child star. She and her twin sister Ashley Olsen shared the role of Michelle Tanner on Full House, beginning at age six month.

E! reports that Samantha Who? producers are happy with Olsen’s performance. No word on whether she might snag a callback for a future episode.

Samantha is played by gorgeous actress Christina Applegate. The show won the distinction of being the highest rated new sitcom in 2007, making it a shoe in to be picked up for a second season which begins in September.

Mary-Kate appeared on the Late Show with David Letterman earlier this week. We’ve got photos of the bohemian waif as she was entering the Ed Sullivan Theater. Unlike her normal demeanor, she actually smiled in a couple of the poses.

Check them photos and Olsen’s Letterman performance video below.

Photos: WENN

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Filed under: Celebrities, Mary-Kate Olsen