Mary-Kate Olsen stars in 'Weeds' Monday night at 9 on Showtime.

The large ensemble cast of Weeds is turning into quite a full house, especially now that Mary-Kate Olsen joins the hit Showtime comedy for a 10-episode run starting Monday, September 17.

Best known for her fluffy child and tween roles with twin sister Ashley, Olsen is confident and funny in her new role as Tara, a born-again Christian who rocks the world of Silas (Hunter Parrish), the elder teenage son of pot-dealing soccer mom Nancy Botwin (Mary-Louise Parker).

"Tara is a good Christian girl from Majestic, the new town next to Agrestic," Olsen says. "She becomes Silas' girlfriend, and they kind of bond over smoking pot, and you'll see what happens to this character with everything that is going on.

"I think you could say that Tara has some different beliefs from the average Christian, though, of what's right and wrong. She wants to tell them all about the Bible. Maybe she's stoned while doing so, but ..."

Former child star

Casting a former child star in a role like this may sound like another gleeful bit ofsubversion for a show that goes sacred cow-tipping every chance it gets, but series creator Jenji Kohan insists that Olsen won the role simply by being very good.

"It was really a situation where she took the part in the (audition) room," Kohan says. "It was really exciting to see. (Mary-Kate and Hunter) read a scene together and it was just so natural and delightful to watch ... We wanted someone really good for this part, and we got that."

Olsen says she had no hidden agenda in pursuing her Weeds gig. She wasn't out to shock anyone, she says. She just wanted a chance to play an interesting role in a buzz-worthy TV series starring two of her favourite actresses - Parker and Elizabeth Perkins.

"I went to NYU for two years and studied acting again," she says, "and that was really when I fell in love with it for the first time and decided that was what I wanted to do."