In her first major solo role without her twin sister, Mary-Kate Olsen is returning to series television for the first time since 2002 with a co-starring role on Showtime’s dark comedy “Weeds.”

Olsen will play Tara, a devoted Christian girl living in the newly developed megachurch community Majestic who becomes a love interest for Nancy Botwin’s (Mary-Louise Parker) son Silas (Hunter Parrish). Olsen will appear in 10 of the 15 episodes of the show’s third season, which premieres August 13.

Since their ...