Ashlie Atkinson, who stars in Marisa Wegrzyn’s dark comedy “The Butcher of Baraboo,” which opens today, isn’t afraid to do what’s best for a character, even if it means writing herself out of a job.

Not long after Ms. Atkinson landed her big break on television — a guest role on the firehouse drama “Rescue Me” — the show’s writer, Peter Tolan, asked Ms. Atkinson and the actor who played her character’s boyfriend, Mike Lombardi, for suggestions on the direction of their relationship. Ms. Atkinson, 29, was playing the role of Theresa, an overweight girl dating a fresh-faced firefighter.

“He said ‘What’s the weirdest thing that you think could happen to you guys? Where could this go?'” Ms. Atkinson said. “Mike said, ‘She could get pregnant.’ And I said, ‘I could. Or I could break up with him because he’s not smart enough.’ And they were like, ‘Oh, that’s kind of awesome.’ Because the world never expects the overweight girl, no matter how cool, and stylish, and talented she is, to dump the good-looking guy, even if he’s dumb as a bag of bricks,” Ms. Atkinson said.

But there was a catch: “Only after did I realize that I had written myself off a hit television show. I don’t know if I’d make the same choice now, but I do like that I made it.”

That earnest choice, while perhaps unusual for an up-and-coming starlet, is par for the course for Ms. Atkinson, who at 5’8″ and 200 pounds has managed to navigate the muddy waters between being typecast and using her physical features to her advantage. In 2005, with just five lines on “Law and Order” and the guest spot on “Rescue Me” under her belt, Ms. Atkinson landed a coveted part: the savvy, self-deprecating title role of Neil LaBute’s play “Fat Pig.” This fall, she’ll appear in writer-director Noah Bambauch’s latest film “Margot at the Wedding” with Nicole Kidman and Jennifer Jason Leigh. When the curtain goes up tonight on “The Butcher of Baraboo,” at the McGinn/Cazale Theatre, she takes on the role of Midge, a sullen, sarcastic drug-pushing pharmacist.

“In some ways she’s so different from me,” Ms. Atkinson said of her character. “I like to observe, but I can’t shut my mouth off a lot of times.”

Indeed, in her offstage and offscreen life, this Little Rock, Ark., native with a smoky voice and red-tinged hair is no less a spitfire. She joined the Gotham Girls Roller Derby in 2003, when the tongue-in-cheek racing was a nine-person pet project. She has since helped build it into a four-team league that competes throughout New York City. Ms. Atkinson, who skates under the pseudonym Margaret Thrasher, “Prime Minister of Your Demise,” lists as her distinguishing characteristics “cigarette surgically attached to upper lip” and “whiskey breath.”

But while she has built a persona on brash, uninhibited choices, she has also chosen to portray characters who, though headstrong, are frequently the recipients of abuse. The quick-witted and sharp-tongued Midge is ultimately a punching bag for her family and acquaintances. “You’re attractive in your own, unique way,” Midge’s mother, Valerie — played by “That ’70s Show” star Debra Jo Rupp — says with a sneer. “If a retarded boy can get married, there’s hope for you.” Later in the play, Valerie clobbers her daughter on the back of the head before hog-tying her and threatening her with a butcher knife.

To Ms. Atkinson, Midge’s position as a receptacle for abuse signaled a juicy part. “Often it’s just a side effect of the interesting role,” she said. “If you take the interesting character role, there has to be a character somewhere in it that opposes you and the way you conduct yourself.”

Ms. Atkinson separates the verbal missiles launched at her characters from thoughts about herself, but the painful dialogue can make for uncomfortable exchanges. The first read-through of “Fat Pig” — the title of which cut deep enough — made for some politically correct tiptoeing.

“I felt like it was a low-rent version of ‘Guess Who’s Coming to Dinner,'” she said of Sidney Poitier’s 1967 race-relations drama. “Where people are dropping the nword, and then looking down the table like, ‘Mr. Poitier, you know, I don’t think that that about you, like, personally.'”

During rehearsals for “Fat Pig,” Ms. Atkinson recalled, the other actors would say their lines, then follow them up with: “We love you. We cherish you. Please don’t think …”

Despite the challenges of portraying the discarded Helen in “Fat Pig” and the outcast Midge, Ms. Atkinson has deep affection for her characters. “I wonder things about Midge. When she’s sitting in the dark by herself, I think she’s smoking an imaginary cigarette,” she said of a scene in which a character stumbles upon Midge, in an easy chair and bathrobe, in a darkened living room.

“Some of these things, you know that no one’s ever going to know,” she said. “You build a life so you can show a fraction of it.”

That sense of exploration was a boon to “The Butcher of Baraboo” director Judith Ivey. “She doesn’t censor a choice until after it’s tried,” Ms. Ivey said of Ms. Atkinson. “She has enormous flexibility, and she’s more instinctual than some actors. Debra Jo has an incredible technical facility and has developed it through the years of her career, and Ashlie works with it so well even though she isn’t as refined a technician. But she’s made herself a student of it.”

Judging by the direction of her career, Ms. Atkinson is a fast learner. But while she may now list actresses like Ms. Kidman and Ms. Leigh as her co-stars, the Roller Derby queen isn’t quite ready to trade in her skates for celebrity status. Ms. Atkinson, displaying some of the self-deprecation so common to her characters, recalled the playwright Ms. Wegrzyn’s comment that “The Butcher of Baraboo” would be the first in a trilogy of plays. “I figure, she’s 26, I’m 29,” Ms. Atkinson said. “If she keeps writing these plays, maybe I can stay employed for a while.”

Until June 30 (2162 Broadway at 76th Street, 212-246-4422).