Every TV show needs an antagonist, and this season on CBS's "The Big Bang Theory" (8 p.m. Thursday, KDKA-TV), that role belongs to former Pittsburgher Aarti Mann.

She plays Priya, the sister of Raj (Kunal Nayyar), who has become the girlfriend of Leonard (Johnny Galecki), much to the dismay of Leonard's ex-girlfriend, Penny (Kaley Cuoco). In the final two episodes of the season, Priya will play a central role, especially in the season finale.

"Basically things between Penny and Priya come to a head," Ms. Mann said in a phone interview earlier this month from the Los Angeles home she shares with her husband, who works in finance. "Priya has to face the music, and she and Leonard come to an important crossroads."

Ms. Mann said the show will end on two cliffhangers, one involving her character, and she doesn't know whether she'll be invited back for the show's fifth season.

"I would love to come back and see where it goes with Leonard and Priya," she said, "but no contracts have been signed. I think we'll know by June or July."

Ms. Mann was born Aarti Majmudar in Connecticut (Mann is a truncated version of her married name) and moved to Pittsburgh as a toddler. Growing up, her family lived in Mt. Lebanon and Wexford before settling in Fox Chapel where her mother, Vasanti Majmudar, an obstetrician/gynecologist out of UPMC Passavant, still lives. (Ms. Mann's father passed away while she was in high school).

Ms. Mann graduated from Shady Side Academy (she declines to reveal the year) and studied film at New York University. She never acted in plays in high school but while in college studying film writing and directing, she helped fellow students on their productions.

"I convinced them to give me a featured extra part just so I could be in front of the camera," she said.

After college she wasn't sure what to do with the degree. She wrote a script for a Pittsburgh doctor who moonlights as a filmmaker.

"I'm the daughter of two Indian, immigrant doctors and I have an older sister and younger brother and none of us have pursued medicine as a career," she said. "We're all over the artistic side of things."

Ms. Mann's brother, Nishad, is a journalist and her sister, Kruti, helped influence her decision to switch to acting. Kruti, who also lives in Los Angeles, is a filmmaker and cast Ms. Mann in her 2006 film "The Memsahib." While filming her scenes, Ms. Mann said she "got bit by the bug" to act and enrolled in acting classes in Los Angeles.

"Our parents were so supportive of us," she said. "My mom always knew I was going to be an actor because I was a ham from the very beginning so she would push me toward it, which is really unconventional for Indian families to do. I would not be doing what I'm doing today if not for the love and support of my mother."

Ms. Mann booked a national commercial for Volvo and other roles followed, including a part in the web/TV series "quarterlife," which aired briefly on NBC in 2008. She was also a guest star on "Heroes" in 2009.

"Each year I've been at this game I've set a goal for myself and I've hit it every single time," she said. "My goal for this year was to get a recurring role on a TV show. We're a quarter into the year and I've already done that."

Someday she might like to return to the behind-the-camera work she trained for in college but for now she's content acting.

"The thing I love about acting is it's all about being in the moment," she said. "That kind of creative process is really what turns me on."

Ms. Mann had not watched "Big Bang" before joining the show and her acting background isn't even in comedy. So how did she end up on one of TV's top-rated sitcoms?

"I'm asking myself the same question," she said with a laugh. "Whenever we're rehearsing I'll try to play it in a more comedic way and [executive producer] Chuck [Lorre] always comes in and says, 'Play it simply. You're beautiful when you're simple.' I'm constantly told to reel it back in. I think I'm giving them what they need for the show. Every sitcom needs their straight man or straight woman."

Ms. Mann appeared on "Big Bang" after series regular Kaley Cuoco broke her leg early in the season forcing a delay in production and re-jiggering of story lines.

"If Kaley hadn't broken her leg, I don't know if Priya would have ever existed," she said. "The writers went into their vault to look for plots that were light on [the Penny] character and they came up with the possibility of exploring Raj's cousin."

When Ms. Mann auditioned for the show, Priya's relationship to Raj was different than the sister she eventually became.

"The character changed completely. Originally she was engaged and she was going to get married and Leonard was her last fling. We rehearsed it over the week and it became just Priya making a booty call," she said. "It was revealed to me after we shot the show that [executive producer] Bill [Prady] wanted to leave the option open for Priya to come back later in the season."

After filming her first episode, Ms. Mann and her husband went to India for seven weeks. When she arrived back in Los Angeles on the drive home from the airport, Ms. Mann's agent called to say "Big Bang" wanted her back on set in three day's time.

After that she was booked on the show one or two episodes at a time.

"I spent this whole arc not really knowing how long I was going to be on or where it was going, which is great because it forces you to play the material as opposed to playing the overriding arc of the character," she said. "It's fun to see how Priya has changed. She's made strong turns and it's been fun to see how fan sentiment has turned with it. There are people who loved Priya and now completely hate her. The character will make yet another turn before the end of the season."

Even Ms. Mann admits, if she looked at the character as only a fan, she might not love Priya.

"But I'm playing her and if I can't find the humanity in her and defend her, no one else will," she said. "Whenever I get the material and see Priya saying things I would never say in my own life, I just have to roll up my sleeves and figure out why she's saying it. She's a flawed human being. ... She's very confident, accomplished and successful and at the same time she's a little insecure about the gorgeous blond woman who spends too much time in the apartment [with her boyfriend]. Who wouldn't be threatened if a Penny was hanging around all the time?"

While she waits to hear whether she'll be asked back to "Big Bang," Ms. Mann hopes to land a dramatic role -- "that involves crying and screaming and turmoil" -- and she wants to get back to her acting roots.

"I'd really love to get back into commercials, actually," she said. "I love how quick and breezy they are. They often give you the freedom to improvise and it's a 30-second story so you're not burdened with creating a multi-layer character, you're just playing an idea or concept. And honestly, they pay really well."