Actors dream of landing that one role that ignites a career.

Even if it’s not quite the “42nd Street” fantasy of instantaneous stardom for the understudy, a supporting role on a TV series can be an important stepping stone, especially if the show is a critical and/or commercial success. And in rare instances, the magic of being in the right supporting role on the right show at the right time can propel a player who’s the second (or third) banana into full-fledged stardom, the kind that inspires Internet parodies, Halloween costumes and social-media shrines.

Such was the case for three actors who rocketed from virtual obscurity, through scene-stealing supporting roles, to become likely contenders in this year’s Primetime Emmy Awards derby. Christina Hendricks made Joan a force to be reckoned with during the pilot shoot for “Mad Men.” Aaron Paul saved the life of Jesse Pinkman by virtue of how compelling he was in the skin of the tortured soul on “Breaking Bad.” And with her first TV role, Allison Williams found her life changed as millions of viewers came to love (or love to hate) her as spoiled twentysomething Marnie Michaels on “Girls.”

The trajectories of the three thesps over the past decade underscores how much the career-building process has expanded for actors. A little sizzle on TV can lead to bigger things in the traditional sense — movie roles, stage work, personal appearances and endorsements — as well as licensing agreements, production deals, Internet ventures, videogame work and even, in Hendricks’ case, a Barbie doll modeled after her character. Call it the new upward mobility for actors with the right stuff.

Of course, success is not without its challenges, and thesps need to move cautiously as opportunities arise, so as to not erode their newfound status.

“You’re constantly pushing the boulder uphill to convince people that what they saw last isn’t the only thing an actor is capable of,” says Dar Rollins, partner and co-head of talent at ICM Partners. “I think being able to show different colors is the way to sustain a long career. It becomes incumbent upon the artist, alongside the representative, to think of outside-the-box ideas for what their next job is.”

The rush of fame is heady stuff to handle. Suddenly, actors who were sweating out jobs from role to role have to make careful choices on everything from talent representation to the many job offers that come flooding through the door. The experience is especially surreal when you’re associated with a show that’s become part of the zeitgeist.

Paul marvels at all the parodies and tributes to “Breaking Bad” he finds online. And how he went from being best known for an arc as Amanda Seyfried’s boyfriend on HBO’s “Big Love” to counting some of the industry’s biggest names as fans.

Two years ago, the night before Daniel Day-Lewis would win his third Academy Award, Paul spotted the “Lincoln” star at a party. “We make eye contact,” Paul says, “and he just bows to me.” The rest, he adds, is a blur, but he recalls Day-Lewis shaking his hand and professing his love for the show. “I forget what I even said,” Paul admits. “I think I went deaf.”

Says Williams of landing the role on “Girls,” which bowed in 2012: “It’s absolutely changed my life.” Prior to playing Marnie, the actress had only minor credits — a couple spots on the NBC series “American Dreams” as a teenager, and appearances in Internet shorts. In fact, it was a 2010 video, in which she sang the song “Nature Boy” set to the music of the “Mad Men” theme, that caught the eye of producer Judd Apatow, who recommended calling Williams in for the project he was developing with writer-director-star Lena Dunham.

Williams had just moved to Los Angeles from New York, and “Girls” was her first audition in town. While she calls the experience “fairly standard,” she was impressed to meet Dunham right away, and she also made a quick observation. “The room was filled with women,” she says. “There wasn’t a single man there.”

Dunham instantly saw something in Williams. “Allison has an unusual, intelligent sense of humor and a general sparkle that is impossible to ignore,” Dunham notes. “I felt excited and inspired improvising with her. Our energies are so different, and I knew that contrast would inform our scenes in an exciting way. Plus, she’s damn adorable.”

Landing the role of Marnie wasn’t just a boost for Williams’ career; it affirmed her life choice. “My first day on set as a professional actor was shooting the ‘Girls’ pilot,” she notes. “It was a big moment that was either going to confirm or deny the lifelong belief I’d held that I was going to be an actress.”

Unlike Williams, both Hendricks and Paul had been working actors for years, racking up guest spots on countless shows and getting their hopes up for busted pilots and short-lived series.

“I’ve been doing this now for 17 years,” Paul says. “There were ups and downs, lots of struggles, but I was content, and happy to be working.” Everything changed when he read for “Breaking Bad” creator Vince Gilligan, who didn’t take long to become a fan. “I was completely unaware of him as an actor, but he was fantastic from the moment he walked in the door,” Gilligan says.

In fact, Gilligan actually had seen the work of Paul, who had appeared in an episode of Fox’s “The X-Files,” on which Gilligan was an exec producer. The actor had played the gonzo producer of a “Jackass”-style show. Laughs Gilligan, “He was so great in that, and he’s such a chameleon, I didn’t recognize him from

my own show.”

Paul was unaware, until halfway through the filming of the initial year of “Breaking Bad,” that Jesse wasn’t intended to stick around long. “The plan was to kill him off at the end of the first season,” Gilligan admits. “I figured that character would have served his usefulness, and his death would drive Walter White onward to even more drama in season two. But very quickly I realized that would be a ridiculous thing to do. (Paul) was so very good and he and Bryan (Cranston) had such great chemistry together — no pun intended.”

Similarly, Hendricks found herself getting upgraded from guest star to series regular, after “Mad Men” creator Matt Weiner tapped her for the pilot. She had fallen in love with the world represented in the script, and didn’t care that the show was on AMC, then an unproven network. “There was another project (at a broadcast network) I was up for that was not nearly as interesting to me,” she notes, adding that it seemed like a can’t-miss proposition. “In the past, I had done series that seemed like the sure thing,” she says, “and all the elements were in place for them to be a hit. And they didn’t go. So I told myself, ‘Do the one you love.’”

Hendricks originally auditioned for the role of Midge, one of Don Draper’s lovers, eventually played by Rosemarie DeWitt. When the casting directors suggested to Weiner she read for Joan, he admits he was surprised. But he came to see Hendricks’ character as compelling in her own right, and a means of adding depth to Elisabeth Moss’ Peggy.

“I had imagined Joan as more bookish,” Weiner says, “someone who would be Peggy’s friend.” But Hendricks was playing the character with more confidence than was written. “It was warmer, sexier, than I imagined it to be,” Weiner recalls. “She had a stronger sense of who Joan was than I did. And that was actually a big turning point for me to juxtapose these characters. Peggy embodied this professional interest, and Joan the Helen Gurley Brown philosophy of ‘I will use anything I can to get what I want — but ultimately what I want is a house in the country.’ ”

With Hendricks, Weiner realized the character had big potential. “After her audition, we asked for the money and placement to make her a series regular, because I knew this would be an amazing way to see Peggy’s story play out, not even knowing yet if Joan had a story,” Weiner says.

By breathing life into roles that become so integral to a show, actors can’t help but influence the development of their characters.

Sometimes, it can be the simplest suggestion that has an impact. Gilligan recalls Paul coming to him after season two and asking if Jesse could cut his hair short. “I don’t know that he ever put it into words, but I think he was sensing that Jesse was at a point in the story where he was very much buying into the cult of Walter White,” Gilligan says. “Jesse was subconsciously trying to look more like his mentor. And it was an excellent detail that did not come from me, did not come from the writers, came strictly from Aaron Paul.”

Because the smallscreen affords actors the opportunity to play a character over more than the two hours of a feature film, it’s not uncommon that fact and fiction begin to merge. Dunham says Williams shares qualities with Marnie, noting, “Allison is unfailingly polite in an Emily Post-approved sort of way, and a bit of a control freak — a fact she herself will hilariously admit to — and Marnie has taken those qualities to the extreme.”

Hendricks also cops to inhabiting her role. “I’m absolutely in Joan,” she says. “We all deny that our characters are anything like us, but of course there are parts (of us in them).” Sometimes, she says, those similarities can come from the creatives observing the people who are playing roles. “It’s hard for writers to know their actors that well and not play to their strengths,” she adds.

Weiner concurs with his actress about the nature of using real life as a muse: Who Hendricks is offscreen greatly informs Joan onscreen. “You start writing things and seeing what fits and who the person starts to be. She’s different from Joan — she’s not as harsh, she’s not a know-it-all. But there’s a lot of Christina in there,” he says. “She’s a much softer and kinder person, and sometimes you see it peek through.”

Paul figures the “Breaking Bad” writers had a different challenge, since Jesse wasn’t intended to last past the first season. “They had a lot to think about once they decided to keep me around,” he says. “I love that they decided to go against the norm, for this kind of druggie, burnout character. You get to see at the root of it all, he is just a mixed up, damaged kid who has his life turned upside down.”

According to Gilligan, Paul is most responsible for Jesse’s dimensionality, and even for influencing the show overall. “There’s an innate sweetness to him as a person that shines through, no matter how villainous a part he may be playing,” Gilligan says. “He was kind of a moral center for the show, and I don’t believe he would have become that if not for Aaron Paul’s (own) strong moral center.”

Coming face to face with the intensity of fan response to a buzzy show can be unnerving, the actors admit. Today’s super-fans don’t just watch a favorite show once a week, they can live with it virtually 24/7 through social media and digital tie-ins.

Paul noticed a response to “Breaking Bad” almost immediately. “It was sudden and strange; we saw it in the very first season,” he recalls. “People would come up, and not just say, ‘Hey I like your show.’ It was people grabbing my shoulders, screaming at me, ‘My God I love your show!’ Each season (the reaction was) just more and more intense.”

Having your private life become fodder for media coverage takes getting used to. “The lack of privacy, the criticism and invasion on people’s lives has quadrupled since I began ‘Mad Men,’ ” Hendricks notes.

Behind the scenes, there’s also a jolt that comes when the phone starts ringing with job offers. That’s a big shift from having to chase down work. After “Breaking Bad’s” first season, Paul was tendered a role in “The Last House on the Left” without an audition.

This year, he headlined his first movie, “Need for Speed.” The DreamWorks pic was based on a videogame, but the marketing was centered on Paul, which marked a new kind of pressure for the 34-year-old actor. The film underwhelmed at the domestic B.O. with about $44 million, but did better internationally, banking almost $160 million.

Paul’s post-“Breaking Bad” career track will focus on more than action blockbusters, he assures. “I don’t care if it’s a small role in a small film or a small role in a big film or a big role in a small film,” he notes. “I just want to do what I’m passionate about.” That includes everything from the indie film “Hellion,” which he also produced, to a role in Ridley Scott’s upcoming “Exodus: Gods and Kings.” He is executive producer on the Netflix animated series “BoJack Horseman: A Tale of Fear, Loathing and Animals,” in which he voices the human friend of a washed-up movie-star horse.

Hendricks began fielding feature offers for the first time early in her run on “Mad Men.” She met with Steven Spielberg on a project, and was impressed when he expressed his enthusiasm for the show. She landed a small part in “Drive” with Ryan Gosling that led to bigger things. “I was in the back of a swerving car on set one day, and between takes Ryan goes, ‘I’d like to direct you in something,’ ” Hendricks says. “I was like, ‘Oh, thanks, you’re not a director, but that’s super sweet.’ A year later, he sent me a script.”

That film was “Lost River,” in which Hendricks stars as a

single mother drawn into a dangerous underworld. Though derided by most critics at Cannes this year, the experimental noir pic has its fans. “We knew when we were making it that it wasn’t for everyone,” Hendricks says, adding that Gosling made the film he wanted and that the scene at Cannes was glamorous and exciting.

Williams, too, has seen Hollywood take notice. And while rumors she was circling the film reboot of Marvel’s “The Fantastic Four” didn’t pan out, she is looking at several future projects.

“Girls,” she says, has allowed her the luxury to be selective — and to choose a path that won’t leave her typecast.

“I don’t want to rush, because I don’t want to (keep playing roles) like Marnie,” she says. “I’m looking for a director to have the imagination to say, ‘I can see you as a Midwestern woman with a meth addiction.’ So doors will open, but you have to navigate them.”