The controversy around “Kick-Ass” began with the film’s R-rated trailer, which hit the Internet in December and showed a masked little girl shooting grown men in the face and using a certain four-letter word most adults rarely utter.

The trailer turns out to have been just a small taste of the deeply subversive movie. “Kick-Ass,” which opened Friday, is about ordinary kids who don homemade costumes and venture out to fight crime, just like superheroes in comic books. The only difference is that teenager Dave Lizewski (played by Aaron Johnson), the film’s eponymous hero, has no superpowers whatsoever. The first time he squares off against bad guys, he is stabbed in the stomach, hit by a car and spends months in the hospital recuperating.

That scene is the first of many in which “Kick-Ass” upends everything you knew about comic-book superheroes, who traditionally triumph over villains by taking the high road. Realism trounces fantasy. Being on the side of the good and lawful just isn’t enough: You need a machine gun or a shotgun or at least a sword or two to make a difference.

“Someone asked me ‘Is this movie a spoof? Is this a parody? What is this?”‘ says director Matthew Vaughn, who raised the movie’s budget (estimated at around $35 million) through private investors after every Hollywood studio turned him down. “And I said ‘This is a post-modern love letter to comic books and superhero films.’ I just felt like comic-book movies were getting a bit generic and stale, and it was time to do something fresh — but, at the same time, not throw the baby out with the bath water.”

One of the major differences between “Kick-Ass” and traditional comic-book movies such as “Iron Man” or “Spider-Man” is that the filmmakers did not have to figure out how to make their characters relatable to the present day. The film was shot while writer Mark Millar and artist John Romita Jr. worked on the books, and the last installment of the eight-issue series was published in January.

Millar, a chief writer for Marvel Comics who is known for hyper-violent, aggressive books, pitched the idea behind “Kick-Ass” to Vaughn before a single panel had been drawn (one of Millar’s previous series, “Wanted,” had been turned into a big-budget Angelina Jolie film in 2008).

“This is the first comic book adapted in the last couple of decades that is first-generation,” Millar says. “Even ‘Watchmen’ was 23 years old when it got turned into a film. The comic book is of the ‘now,’ which is part of the reason why the movie feels so fresh and daring.”

Millar says one of his creative inspirations was a documentary interview with Quentin Tarantino in which he praised the 1931 classic “Frankenstein.”

“Tarantino was talking about how when that movie is funny, it’s ‘really’ funny, and when it’s scary, it’s ‘really’ scary. He did the same thing in ‘Pulp Fiction,’ when you’re laughing at John Travolta and Samuel Jackson chatting in the car, and suddenly his gun goes off and the guy’s brains are splattered all over the back seat. I always try to do that with my comics, too. You only have 22 pages to tell your story, so every line has to mean something. You try to get the maximum emotion out of every bit of your construct.”

But although “Kick-Ass” was a hit in print, the comic was not universally beloved.

“The book sold a lot, but a lot of people complained about it,” says Michael Avila, a contributor for newsarama.com and msnbc.com who specializes in pop culture. “Millar comes up with great ideas and gets the attitude just right, but the downside is you don’t always like his characters. That’s why ‘Kick-Ass’ is one of the few comic-book movies that actually improves on the book. The movie does to the comic book what Francis Ford Coppola did to Mario Puzo’s ‘The Godfather.’ “

DEPARTING FROM THE BOOK

The script for “Kick-Ass,” written by Vaughn and Jane Goldman, differs from the comic in small but critical ways. For example, the character of Big Daddy, a masked vigilante who raises his 11-year-old daughter Mindy to become a ruthless crime fighter named Hit Girl, is somewhat twisted and deranged on the page. In the film, Big Daddy (Nicolas Cage) is still nuts, but his relationship with his daughter (Chloe Grace Moretz) is tender and touching. Yes, he gives her a butterfly knife as a birthday present and teaches her how to use riot gear by shooting her in the chest. But they are hugely likable psychos, in a Bruce Wayne kind of way.

Although Hit Girl is a supporting character in “Kick-Ass,” she has, because of her age, become the film’s breakout persona — and the magnet for much of its controversy. Children will automatically be drawn to her, but the R-rated movie is not kid friendly, one of the key reasons Hollywood studios initially passed on the project.

“Matthew and I were very keen to avoid any sense of deliberately provoking or shocking the audience,” says Goldman, who previously collaborated with Vaughn on the much-gentler 2007 fantasy “Stardust.” “One thing that kept coming up in our conversations is how young Hit Girl is. But it’s precisely her age that makes her such a fascinating character.

“For me, as a female screenwriter, I found the idea absolutely exhilarating: Here is a strong, female anti-hero, but she’s not sexualized,” Goldman says. “She’s not there for guys to get their rocks off. She’s genuinely dangerous. It was never our intention to shock people by having a kid killing people. If anything, we toned certain things down from the comics. But this was never meant to be a movie for children.”

For Vaughn, the main appeal of “Kick-Ass” was in taking familiar comic-book tropes and placing them in a realistic setting, then standing back to see what happened.

“There isn’t a single thing in this movie that couldn’t happen in real life,” Vaughn says. “That was my rule. All the costumes were made of materials that could be bought on the Internet. Big Daddy’s costume is made up of French riot-police gear. You can go online, search for jet packs and find a guy in Mexico who makes them. You can buy the bloody thing online! When it came to Hit Girl, I told the fight choreographers I wasn’t interested in going too over the top. With a few things she does, they said ‘Look, there’s only a one-in-a-thousand chance this could actually happen if we did it for real.’ But that was good enough for me, because there is still some reality to it.”

THE ‘SHOOT FIRST’ LESSON

The strict adherence to realism inevitably drew an R-rating, because teenagers swear, they occasionally do drugs, and if they leaped off a roof wearing a cape and thinking they could fly, they would plummet and land in a bloody heap.

When Spider-Man or Superman dispatched muggers or drug dealers, they usually delivered them to the authorities, bruised but alive. In “Kick-Ass,” the heroes quickly discover the only way to stop the guy coming at you with a gun is to shoot first.

“I’ve always thought there was a very thin line between bravery and stupidity,” Vaughn says. “Depending on how you look at it, those soldiers in World War I climbing over the trench and running toward the machine guns were very brave but also very stupid. That’s also what Kick-Ass is: He’s brave in one way and stupid in another. He’s also naive and optimistic. It takes a little bit of all of that for heroes to be created.

“To show that, though, you have to go all the way. I’ve always felt movies should be either PG or R. PG-13 is this sort of no-man’s land. Films can imply some pretty horrible violence without showing it. I think ‘The Dark Knight’ was darker and more violent than ‘Kick-Ass’ in a psychological sense. When the Joker started playing with his knife, it made you look away from the screen. In ‘Kick-Ass,’ you’re laughing at the violence and enjoying it for its silliness. You’re not thinking ‘Eeww! That’s disgusting!’ “

The primary challenge facing “Kick-Ass” now is to reach the adult audience that will best savor its humorous exploration of the nature of heroism. The ordinary moviegoer who isn’t plugged into the comic-book world and just glances at the film’s poster in a theater lobby could dismiss it as a picture for 12-year-olds. But its wry sense of humor and cheerfully gory violence is intended for a more-mature, sophisticated palate.

Romita, the famed comic-book artist who drew the “Kick-Ass” series (and served as a consultant on the film), admits that blood is a selling point of either incarnation of the story.

“We went a lot further in the comic than we did in the film,” Romita says. “I’m getting older, and I’m raising my son, so I understand the questions the movie raises. At what point are we going too far? Conservatives say the bar has been raised too high, and society is falling apart because we have no morals. Liberals say let’s progress naturally and be the arbiter of our own affections and let the individual be the bar. I think both sides are right.”

Despite the marketing challenges, “Kick-Ass” generated a bidding war after Vaughn previewed some footage at last summer’s San Diego Comic-Con, with many of the bidders the same people who had originally turned the film down. Lionsgate was the eventual victor.

“I never really sat down in a room and discussed the project with anyone, because they all said no right away,” Vaughn says. “The violence and Hit Girl were always concerns. What made me laugh is that after the studios saw the film, they were all like ‘God, we want more Hit Girl!’ I’m so glad they all said no the first time around, because I don’t think the movie would have turned out as well without that.”