– Lyric Lewis has graduated from St. Paul’s Central High School to sitcom star. But during a recent afternoon on Melrose Avenue, she was one-half of the unheralded lounge act Ketchup and Mustard, grinding out an erotic ode to hot dogs.

The completely improvised performance elicited guffaws from the fewer than 100 high school students attending this special matinee at the Groundlings, a legendary Los Angeles improv theater. It’s a far cry from the millions who will tune in Thursday to watch the rapidly rising actor play the unflappable teacher Stef Duncan in the second season premiere of NBC’s “A.P. Bio.”

But Lewis has no intention of curtailing her commitment to the theater she fantasized about joining while selling clothes at the Woodbury Lakes location of Express.

“If I hadn’t trained in improv, I don’t know if I would have felt true liberty and confidence in myself,” Lewis, 33, said backstage at the venue that also nurtured the careers of Melissa McCarthy, Kristen Wiig and Will Ferrell.

“L.A. is so good at eating you up and spitting you back out without giving you anything. You go on a billion auditions and you don’t hear anything, but you can come and do sketch shows at night and make a hundred people laugh and tell yourself, ‘I’m funny.’ ”

“A.P. Bio” creator Mike O’Brien’s scripts don’t allow his cast much room to ad-lib. But he believes Lewis’ time at the Groundlings was instrumental in preparing her to play opposite veterans such as Patton Oswalt and “It’s Always Sunny in Philadelphia” veteran Glenn Howerton.

A.P. Bio When: 7:30 p.m. Thursdays.• Where: KARE, Ch. 11.

He cites a scene in an upcoming episode, where Duncan rebukes the advances of a moony-eyed student, but is reluctant to return the pricey purse he gave her.

“She’s not rolling her eyes or going for a joke. She’s just listening to him and saying things quietly under her breath,” said O’Brien, a former “Saturday Night Live” staff writer. “It’s funny, because it looks so real. That level of acting is often developed by doing thousands of improv scenes.”

Commanding the stage

Lewis caught the acting bug in her native New Orleans, where she spent summers with a children’s theater camp. When she was 12, her mom, Verona Mitchell, decided to move with Lewis and her three younger siblings to the considerably safer Twin Cities. Lewis was devastated.

“It’s a bummer for any kid to get ripped apart all of a sudden from what she knows, even if we had bullets flying over our heads,” said Mitchell, who was recently divorced with an 8-month-old at the time of the move. “You don’t understand getting into a U-Haul and coming to a place where you don’t have any friends. Plus, I ended up working the night shift at the Target Midway. Guess who was the babysitter? Lyric. It was tough for her.”

Mitchell tried to inspire her oldest daughter by insisting they see plays at the Guthrie and Penumbra. Lewis also got involved in the theater club at Central High School, headed by Jan Mandell, a teacher who inspired other stars including "Superstore” cast member Colton Dunn and stand-up comedian Nick Swardson. Visits from local performers such as T. Mychael Rambo left an impression, as well.

“That’s really when the light bulb went on,” Mitchell said. “When she got on that stage at Central, she commanded it. I could see it in her eyes. It was almost like a possession.”

‘I wanted to be goofy’

After studying theater at Syracuse University, a substitute teacher introduced Lewis to “clown acting,” an early form of improvisation. She was hooked.

“The training at Syracuse was so dramatic,” Lewis said. “But early on, I wanted to be goofy and make people laugh all the time. I started doing research and realized I wanted to move away from the weather in New York and that everyone I thought was dope came from the Groundlings.”

Lewis returned to Minnesota for a year, saving wages earned from her shifts at Express until she could afford a move to Los Angeles. When that day came, one of her first stops was the Groundlings School.

“When I auditioned, all of my belongings were still in my car,” said Lewis, who spent her first few months in L.A. crashing on couches, sleeping in shifts at a friend’s apartment or holing up in her car.

Over the course of the next 10 years, Lewis slowly moved her way up the ladder at the Groundlings, impressing veterans along the way.

“She’s totally fearless, and her characters are always fighting so hard for what they want and believe in,” said longtime teacher and cast member Alex Staggs.

He was particularly impressed by a sketch she co-wrote with Kiel Kennedy. Lewis plays the sassy mother of a trick-or-treater trying to work up the courage to approach a candy bowl guarded by a ­horror-movie extra.

“On the page, it was pretty simple, but I knew when she and Kiel pitched it, there would be magic on stage,” Staggs said. “Because she’s so vulnerable, everyone is immediately invested in her and is willing to join her on her journey.”

Groundlings players don’t get paid, which means they pin their hopes on being discovered by Hollywood power brokers. For Lewis, that moment came when talent manager Naomi Odenkirk (wife of "Better Call Saul" star Bob Odenkirk) caught one of her shows.

Showcases at comedy festivals and guest spots on TV series followed. In addition to her regular role on “A.P. Bio,” Lewis is a semiregular on FX’s “Baskets” and Comedy Central’s “Drunk History.”

“I’m just so happy I didn’t give up. I never came close, weirdly,” said Lewis, who plans to start teaching advanced classes at the Groundlings this spring. “I just had faith it would work out.”