After a half-hour conversation with actress Melissa McCarthy, one might be inclined to assume she’s the most level-headed, regular person on planet Earth.

She’s the sort of woman who plans to make “a big shark cake” for her elder daughter’s fourth birthday and who actually uses the word “gosh” in a sentence while discussing someone who publicly insulted her. She even grew up on a farm in Plainfield, Ill., a place that clearly deserves an award for Most Normal Sounding City in America.

But somewhere beneath all that vaguely Midwestern-accented pleasantness lurks an unhinged, bizarre woman.

The movie “Bridesmaids” – which opens today and stars McCarthy in a performance that achieves new heights in freaky-deaky comedy – is evidence of that fact. It’s also evidence that McCarthy, a veteran TV actress and bit player in studio films, may finally get recognition as one of Hollywood’s most fearlessly funny ladies.

“The women I really love to portray are kind of extreme, fringe, always confident but like, off,” says McCarthy, 40, calling from a Ritz-Carlton in New York while on the “Bridesmaids” promotional circuit.

So off, in fact, that McCarthy – known from her work on shows such as “Gilmore Girls,” “Samantha Who?” and, more recently, CBS’ “Mike & Molly” – was a little worried about reading for the part of Megan, the nuclear engineer in “Bridesmaids” who steals puppies, has no inside voice and thinks “Fight Club” would make a totally solid theme for a bridal shower.

“I went in and auditioned and thought, ‘Oh, I’m probably going to be too weird,’ ” she says. “But I had such a sense of her immediately. Then I thought, ‘I’m going to do it how I want to and most likely they’re going to say, “Thank you. That was super-weird. Please leave.” ’ ”

But “Bridesmaids” co-writer Annie Mumolo – who, like McCarthy and many of the women associated with “Bridesmaids” was part of the Los Angeles improv troupe the Groundlings – says McCarthy nailed it: “When you have the abilities that Melissa has, she can get as weird as it gets. But she can also play Sookie St. James on ‘Gilmore Girls.’ When you have that range, it’s like, where’s the sweet spot for this character? Where is it in the range? She landed right on it.”

Playing the role opposite fellow Groundling alums such as Kristen Wiig and Maya Rudolph called for lots of improvisation. It also required her to embrace physical comedy in a way that is routine for the Jack Blacks and Kevin Jameses of the world but rarer for a woman.

When McCarthy lifts up her leg and starts caressing her calf in an attempt to recruit an air marshal into the Mile High Club, it plays like a triumphant, albeit really odd, moment for summer-movie female empowerment. (That air marshal, by the way, is played by McCarthy’s husband, actor Ben Falcone.) “She’s not very middle-of-the-road,” McCarthy says of Megan. “I feel confident in saying that.”

While McCarthy always enjoyed making people laugh – she remembers singing silly songs as a child for her mother, then demanding detailed critiques – she originally wanted to become a fashion designer. She majored in textiles at Southern Illinois University and eventually found herself designing costumes for plays.

“I think that was the first time I realized, ‘God, I love the minutiae of a character and every little thing.’ ” she says. “What do they read? What do they carry? Are their glasses on a chain?”

She loved it so much she started adding unsolicited elements to some of the actors’ costumes. “I’d give them the pants, and I’d be so excited about it,” she recalls. “If they were doing a period piece, I’d be like, ... ‘I put some change in (your pocket) and it’s all from the right time period.’ And they were like, ‘Can I just have my pants?’ ”

Eventually a friend persuaded her to move to New York. And that same friend urged her to perform at an open-mike night, which she eagerly did despite having zero prepared material.

“I didn’t know what the light was for,” she says, referring to the signal that tells comics it’s time to wrap up their stand-up. “It was the worst thing. They kept flashing the light in the back of the room. Which, every time they flashed it, I thought they were saying, ‘You’re doing great!’ So I’d launch into another story.”

Still, she did well enough to continue on the stand-up circuit and, eventually, relocate to L.A. She began landing parts on television and in movies such as “Charlie’s Angels” and “White Oleander.” (Her first role: a character named Melissa on her cousin Jenny McCarthy’s sketch comedy show.)

If nothing else, she says, she hopes the unconventional chick flick will broaden the kind of female roles that typically populate Hollywood comedies.

“So often I watch a movie and I’m like, who are these women?” she says. “Who’s crying over, like, a shoe sale?”