‘Weeds': Michelle Trachtenberg Debuts Monday as Pot Dealer (Video)

In other Showtime news, “The Big C” gets an all-day marathon

To paraphrase Vince Vaughn in "Swingers": Our little Michelle Trachtenberg's all growns up.

The actress is now 25, but we still remember her best as Sarah Michelle Gellar's younger sister on three seasons of "Buffy the Vampire Slayer" and from her role in the 1996 kid comedy-thriller "Harriet the Spy."

Also read: Michelle Trachtenberg to Go to Pot on 'Weeds'

But for two upcoming episodes of "Weeds," Trachtenberg will play a bona-fide adult. She debuts Monday, August 15, as Emma, a low-level pot dealer who becomes, according to a Showtime release, "an unexpected new adversary" for Nancy. (See the preview clip for the episode below.)

Showtime also announced Wednesday that it will be airing a marathon of the seven episodes of "The Big C" that have aired so far this season. They'll air back to back starting at 2:30 p.m. Despite pulling down solid ratings, the hit drama-comedy, which stars Laura Linney as a Minneapolis teacher diagnosed with Stage 4 melanoma, still awaits an order for a third season.