The scene they were filming on their action comedy “The Heat,” in which Ms. McCarthy and Ms. Bullock play mismatched law-enforcement officers, was a seemingly straightforward one in which the actresses would try to force themselves through a doorway at the same time. But that’s when Ms. McCarthy saw the opportunity to have some fun with her co-star and decided to get aggressive.

As a calmer, gentler Ms. McCarthy would later recall the moment: “I thought, I may just have one shot at this. I’m going to push Sandy Bullock as hard as I can. I’m, like, really, really shoving her.”

The seemingly fragile Ms. Bullock did not just stand there and take it, either. “She’s tiny but she’s mighty and she would push back,” Ms. McCarthy said. “It was a true fight to the death.”

Ms. McCarthy, too, can be deceptively delicate. In person she is still the ingenuous Midwesterner who never totally left the farm she grew up on, and not far removed from the straight-talking, good-hearted wife and teacher she plays on the CBS comedy “Mike & Molly” (for which she won an Emmy Award).

But when Ms. McCarthy is given the ball and allowed to run with it, she will take it to the end zone, the stadium parking lot and the next town over.

Her total commitment and lack of self-consciousness have defined some of her best known performances, like the man-hungry, gun-loving gal pal she played in “Bridesmaids” (which earned her an Oscar nomination); similar qualities also come through in the standoffish but highly capable detective she plays in “The Heat,” which Fox will release on June 28.

In her unassuming way, Ms. McCarthy has quickly earned a freedom to play a wider range of characters than her female peers, and to play them as hard, crude and over-the-top as her male counterparts. Her rising celebrity means she can have roles rewritten for her and movies green-lighted by signing onto them; it has also made her a target for some unexpected and shockingly personal criticism.

But Ms. McCarthy isn’t looking to be a pioneer any more than she wishes to be a punching bag: What she wants from her comedy is the chance to play in a world without consequences.

“You push so far past the normal boundaries of what’s O.K. in society,” Ms. McCarthy said excitedly over a lunch in April, on a trip to New York to host “Saturday Night Live.” “I’m always fully aware of, ‘You can’t do this.’ ”

“When someone really believes in what they’re saying, but it’s crazy,” she added, “it’s like my favorite thing on earth.”

A few days later, Ms. McCarthy was rehearsing on the “Saturday Night Live” stages at NBC’s Rockefeller Center studios, gingerly trying to figure out how best to get her head stuck in a piece of scenery for a sketch that, alas, would be cut before the live broadcast.

For someone who much of America has seen spout R-rated innuendos and douse herself in ranch dressing, Ms. McCarthy could still sometimes seem like a giggly fangirl, laughing at cast members like Taran Killam and Vanessa Bayer instead of playing the sketch with them. But by Saturday night, she’d be teetering around the studio in too-tall high heelslike a pro.

Backstage, Ben Falcone, Ms. McCarthy’s husband and frequent collaborator (he plays the soft-spoken air marshal her character falls hard for in “Bridesmaids”) was explaining how, while his wife’s career prospects had changed dramatically since that 2011 film became a $288 million worldwide hit, her approach to her work had not.

“We’re not very far removed from having to take any job that we can find, to get a job, because you want to be working,” Mr. Falcone said. “Someone sends you a script for the craziest idea ever — it could be about, like, the Abominable Snow-Woman — and you could be like, ‘I don’t know, maybe we should consider it?’ ”

Before her surging stardom, Ms. McCarthy, 42, was a tennis-crazed teenager growing up in Plainfield, Ill., and a 20-something New Yorker paying her dues in stand-up comedy and in what she described as “very dramatic plays, very far off Broadway.”

At a turning point in the late 1990s, she moved to Los Angeles and joined the influential comedy troupe the Groundlings. There, Ms. McCarthy learned the difference between characters who were eccentric and those who were flat-out crazy.

“Crazy’s just crazy and there’s nowhere to go,” she said. “You can have a point of view, it can be very strange, but we have to know your reasoning.”

Training with future stars like Kristen Wiig and Maya Rudolph, Ms. McCarthy also met Mr. Falcone, who busted her in a Groundlings class for improvising a monologue she’d pretended to write on paper.

While her “Groundlings” classmates went off to “Saturday Night Live,” Ms. McCarthy became a supporting player on “Gilmore Girls,” as the embarrassment-prone Sookie St. James, and, in 2010, landed her lead role on “Mike & Molly.”

The following year, Paul Feig, the director of “Bridesmaids,” and its producer Judd Apatow were struggling to cast the character of a goofy friend to Ms. Wiig (who wrote the film with Annie Mumolo).

Mr. Feig said this character was written “to be very nervous and high strung, that friend who’s always falling apart, or at least just high maintenance.” But when Ms. McCarthy came in, wearing Guy Fieri-style bowling shirts and playing her rough and overconfident, Mr. Feig said, “It’s the funniest take on it I’ve ever seen.”

Then as she was wrapping “Identity Thief” in Atlanta last summer, Ms. McCarthy signed on for “The Heat” — barely two weeks before it started production in Boston, when it was called “Untitled Female Buddy Cop Movie” — knowing that Mr. Feig and Ms. Bullock had signed on.

“Identity Thief” became another box-office hit in February, but a particularly scathing review of the film drew national attention. Writing about the moviefor The New York Observer, Rex Reed described Ms. McCarthy as “tractor-sized” and called her “a gimmick comedian who has devoted her short career to being obese and obnoxious with equal success.” Though this review was widely pilloried for its excessive cruelty, Ms. McCarthy did not immediately respond to it.

When Ms. McCarthy was asked about the review over lunch in April, her characteristically cheerful tone evaporated. In a softer voice, she said her initial reaction to reading it had been “Really?” and then, she said, “Why would someone O.K. that?”

Without mentioning the name of its author, Ms. McCarthy said: “I felt really bad for someone who is swimming in so much hate. I just thought, that’s someone who’s in a really bad spot, and I am in such a happy spot. I laugh my head off every day with my husband and my kids who are mooning me and singing me songs.”

Had this occurred when she was 20, Ms. McCarthy said, “it may have crushed me.” But now, as a mother raising two young daughters in “a strange epidemic of body image and body dysmorphia,” she said articles like that “just add to all those younger girls, that are not in a place in their life where they can say, ‘That doesn’t reflect on me.’ ”

“That makes it more true,” she said. “It means you don’t actually look good enough.”

Ms. McCarthy was about to say more when the restaurant began a long and very loud test of its fire alarm.

“I imagine that’s my publicist,” she said after a tension-breaking laugh. “The gods didn’t want us discussing this.”

For Ms. McCarthy, “The Heat” was, like “Bridesmaids,” an opportunity to build a character from the ground up, to give her frazzled, Patti Smith-style hair and an out-of-date wardrobe, and to get to shoot what she called “a crazy machine gun.”

For Ms. Bullock, who plays a strait-laced F.B.I. agent, it was important that “The Heat” be a movie where women played roles that could just as easily have gone to men, and could be a rare female-led film during the film industry’s period of peak testosterone.

“You don’t really see lady-times taking on the summer schedule,” she said.

But the greatest challenge they faced on “The Heat” was a ticking clock and what Ms. Bullock called the “Wild West mentality” of its hectic production.

“You didn’t have time to let any insecurity creep in,” said Ms. Bullock, an Oscar winner for “The Blind Side.” “The first day was like, ‘O.K., we’re not saying anything that’s on the page.’ Then we get into a slap fight.”

In these comic confrontations, Ms. Bullock said of Ms. McCarthy: “She will go for the jugular and, if you’re delicate about it, you’re going to sink. So you fire back — you go for the carotid artery.”

Mr. Feig said of Ms. McCarthy that it was “fun to watch her abuse America’s sweetheart,” but also to see how much more confident of a performer she’d become since “Bridesmaids.”

On that earlier film, Mr. Feig said, “she came in with all this inventiveness and creativity, but she was one of the players.” What she now shares with other movie stars, he said, is that they “know how to shine — they know what they want to do and don’t want to do, and they know how to take something that isn’t in their personality and do it better.”

Given the fickleness of the film industry, Ms. McCarthy said this could be her “only time to really pick things that I 100 percent connect with.” If her hot streak should fade, she said she was prepared. “I’ve been trying to play old-lady parts since I was in my 20s,” she said, “so I look forward to all of that.”

For now, Ms. McCarthy has enough clout to spend part of her summer break making “Tammy,” a comedy she wrote with Mr. Falcone and which he is directing, in which she plays an irresponsible woman on a road trip with her tough-as-nails grandmother (Susan Sarandon). Then she goes right onto another film, “St. Vincent De Van Nuys,” which pits her against Bill Murray, one of her comedy idols.

Ms. McCarthy previously got to work with another personal hero, Jane Curtin, who plays her mother in “The Heat,” and who, as a founding cast member of “Saturday Night Live,” taught her that female performers need not depend on others to create their material.

Watching Ms. Curtin with her “S.N.L.” co-stars, Ms. McCarthy realized, she said: “They’re the ones making it. They’re not just saying funny things — they’re actually funny.”

Ms. McCarthy tried to communicate something like this to Ms. Curtin on what she believed was their last day of filming together, but instead she unexpectedly burst into tears.

“I was like, get it together,” Ms. McCarthy recalled. “Don’t always be weird.”

Ms. Curtin then reminded Ms. McCarthy that she still had one more day of work.

“She was really nice about it,” Ms. McCarthy said. “But I was like, I can’t get out cool. I can’t do it.”