Mary-Kate Olsen Guest Stars on Samantha Who?

Mary-Kate Olsen

Billy Farrell/PatrickMcMullan/Sipa

06/24/2008 AT 7:40 PM ET

Mary-Kate Olsen spent her second day on the set Tuesday of ABC’s comedy Samantha Who? for a one-episode appearance this fall, the network confirms.

Olsen, 22, plays a self-destructive bad girl who reminds Samantha (Christina Applegate) of her younger self, but on the set she was all business, Executive Producer Don Todd tells PEOPLE.

“She showed up alone, ready to work, is great on set and is serious about the job,” says Todd. “This is a girl who’s been in front of the camera almost literally her entire life. She’s smart, and she knows what she’s doing.”

Olsen has been stepping out in comedies as of late – she starred as Tara Lindman on Showtime’s Weeds last year – and can be seen opposite Ben Kingsley in the film The Wackness, which hits theaters on July 3. –Pamela Warrick


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