The once-gawky British actor buffed up to play Adam Warlock in “Guardians of the Galaxy Vol. 3.” But he says, in his head, he’s still 5-foot-4.Even when people don’t know Will Poulter’s name, they recognize his face. It helps that the 30-year-old Brit has been acting for half his life and has racked up an eclectic list of film credits, though he’s also blessed with a pair of distinctive eyebrows that are as curvy and expressive as a fleur-de-lis. They pull people in, even if those people aren’t always sure where to place the on-the-cusp actor.“To be honest,” Poulter said, “the bulk of my interactions are, ‘Do I know you from somewhere? Are you the guy from that thing? What have I seen you in?’”Often, this forces Poulter to cycle through a list of his projects until something clicks. Do they remember him as the shy dork who received kissing lessons from Jennifer Aniston in “We’re the Millers,” or the brash friend who meets a bad end in “Midsommar”? Or maybe they grew up on some of the YA franchises he co-starred in, like the “Maze Runner” series and “The Chronicles of Narnia: The Voyage of the Dawn Treader”?Poulter is a patient man, but his willingness to oblige a stranger can still lead to some awkward moments. “No one wants to be put in a position where you’re reciting your C.V.,” he said. Likening himself to a supporting character from “The Simpsons,” he added: “I often feel like I’m doing a Troy McClure impression: ‘You may know me from such things as…’”After this weekend, Poulter’s “where do you know me from” conversations will receive a cut-to-the-chase trump card: He’s joined the Marvel Cinematic Universe, playing the caped superhero Adam Warlock in “Guardians of the Galaxy Vol. 3.” Described in the comic books as a genetically engineered perfect being, Poulter’s Warlock has glittery-gold skin and dangerous powers: Imagine an Oscar statuette that can shoot cosmic beams out of its hands, and you’re halfway there.Poulter as Adam Warlock in “Guardians of the Galaxy Vol. 3.” Whether he returns to the Marvel Cinematic Universe depends on fan reception.Jessica Miglio/MarvelIntroduced flying through outer space to the stirring guitar rock of Heart’s “Crazy on You,” Warlock is a significant figure in Marvel lore, though he’s still coming into his own when we meet him in the new “Guardians” film: Ejected from his birthing cocoon a bit too early, Warlock has a sense of right and wrong that is up for grabs, which gives Poulter several surprising beats to play as he butts heads with the Guardians and considers joining their side.“He brought life and reality to someone who is essentially a child in the body of an adult,” said the film’s writer-director, James Gunn, who picked Poulter over a wide field of hot Hollywood hopefuls. “And,” Gunn added, “he got yoked.”Ah yes, the great yokening. Though he was often cast as scrawny geeks earlier in his career, Poulter’s been through a recent, gym-aided glow-up: 6-foot-2 and Marvel-muscular with a thick head of blond hair, he has followed in the path of fellow British actors Nicholas Hoult and Dev Patel, who played realistically awkward teenagers onscreen before blossoming into Hollywood heartthrobs.Just a few years ago, Poulter was bullied on social media for his looks, but after his physical transformation, he’s been the subject of thirst tweets and internet-boyfriend articles. It’s enough to give a guy whiplash, and Poulter said he’s parsing the head trip.“It’s quite odd, because I’ve sort of formed my personality around looking a certain way,” he admitted. “Psychologically, I’m still 5-foot-4 because that’s what I was at school. Even being tall is something that I’m still getting used to!”Poulter is polite and humble without a trace of former-child-actor neediness. In early March, when I met him for strip-mall soul food in Los Angeles, he had gotten up early to watch an Arsenal soccer game and was eager to follow the match with a big bowl of jambalaya. “Will is completely easy, listens to everything, and is simultaneously very serious and game for anything,” Gunn said. “He’s down to earth and just plain fun to be around.”And though Gunn selected him to play a golden god, Poulter is too self-deprecating to let that kind of role go to his head.“I knew when I was cast that they were definitely going in a different direction than ‘perfect man,’” he told me, grinning.“Will is completely easy, listens to everything, and is simultaneously very serious and game for anything,” James Gunn said. Rosie Marks for The New York TimesTHOUGH IT CAN come with its own special baggage, Poulter has always considered acting to be a safe space. As a preteen growing up in Hammersmith, London, he would spend his entire school week looking forward to drama class on Friday morning, a place where he could kick off his shoes and explore creatively.When he was 12, his drama teachers encouraged him to audition for the charming indie comedy “Son of Rambow”; he landed the film’s breakout role on his first try and filmed it for eight weeks during his summer holiday. “For that to be my introduction to the film industry, I couldn’t have asked for a gentler, nicer, more wholesome experience,” he said. “It really lit the fire in me to want to do it again.”Poulter has worked steadily ever since — you may have also seen his supporting roles in prestige dramas like “The Revenant” and “Detroit” — while also navigating the unique challenge of growing up in the public eye. At 19, his role as awkward virgin Kenny in “We’re the Millers” elevated his profile but led to an uptick in jeers and catcalls from strangers; later, after playing a bespectacled computer-game designer in the 2018 “Black Mirror” episode “Bandersnatch,” some social-media users made such cutting comments about his looks that Poulter announced he’d be stepping back from Twitter to preserve his mental health.That’s why, now that the tide has turned toward appreciative tweets instead of cruel jokes, Poulter is skeptical about putting any stock into what social media has to say about him. “It shouldn’t inform how I treat myself, because I don’t know those people,” he said. “One of the dangers with social media is we can conflate things that exist online to the real world without even questioning it. We just carry the one and don’t really ask whether it actually adds up at the end of the day.”He smiled. “That’s a bad math analogy from someone who’s heavily dyslexic.”He’s seen tweets that compare pictures of his gawky character from “We’re the Millers” to his modern-day, muscular incarnation, as though they couldn’t possibly be the same person. “People are acting like I played Kenny Miller in 2013 and then woke up and now I look like I do, like there was some strange and mystical explanation behind it,” he said. “I just grew up, like every other human being on Earth.”But unlike Adam Warlock, who emerges from his birthing cocoon with a perfect physique, Poulter’s new look took time to attain: He began lifting weights at the start of the pandemic and found the regular fitness regimen did wonders for his mental health. A looming shirtless scene in the Michael Keaton-led limited series “Dopesick” spurred Poulter to step up his workouts, and by the time he began auditioning for “Guardians,” he had already reached the sort of shape that meant he could plausibly play a superhero.“If you want to do it in a way that’s safe and is entirely natural, you have to be prepared to spend a long period of time doing it,” Poulter said. “There’s no way that I could’ve got into the shape that I got had I not been working out for a number of years prior and built up foundations.”Though social-media posts now thirst for him, Poulter is skeptical: “It shouldn’t inform how I treat myself, because I don’t know those people.”Rosie Marks for The New York TimesIf people think his physical transformation happened overnight, Poulter worries they’ll believe he turned to enhanced means to attain it. “Obviously, there’s a lot of pressure out there on young people, both men and women, regarding body image,” Poulter said. “I’m being kind of careful in the words, but if you’re going to promote the process by which you achieved said body goal, I think you have to be fully transparent about how you got there.”Are other actors less than transparent about getting yoked? “Potentially,” Poulter demurred. “It’s not for me to say.”Still, even if Poulter took the long road to his Marvel musculature, he knows it hasn’t stopped people from speculating. “The rumor mill was mad,” he said. “My own mum was sending me something from someone being like, “Has Will had plastic surgery?’”Though Poulter tries to brush all that off, one viral clip still gnaws at him: On YouTube, a physical trainer analyzed a shirtless photo of Poulter from “Dopesick” and criticized his team on the assumption that they had trained him to diet in a certain way.“It’s got millions of views,” Poulter said. “Does it bug me that anyone might believe that, or think that I went about it in a different way that would contradict what I’m an advocate of? For sure. But I guess it’s about learning to relinquish your control over that sort of thing and just hope that there’s enough people who know what’s up.”As we finished lunch, Poulter chatted with our server; over the course of our meal, I had watched it dawn on her that she knew who he was. “You’re very funny,” she eventually told Poulter, who thanked her.We discussed his impending worldwide press tour for “Guardians,” though Poulter said he genuinely didn’t know whether Marvel had bigger plans for him beyond this film: “It kind of hinges on how people respond to the character,” he said. “If the fans don’t like Adam Warlock, obviously I’m going to be pretty gutted. My family’s opinion means a lot, but it’s not necessarily going to bring me back as the character.”But even if it proves to be a one-off, playing Warlock was a valuable experience, Poulter said. When he first started on the production, Gunn told him that he shouldn’t be afraid to screw up, even if those mistakes might make him feel self-conscious. For someone who struggles with how he can be perceived, that advice was scary but also freeing: It meant that he could take big swings and feel safe, and that he could learn to forgive himself when things didn’t go to plan.Those are the sort of realizations that keep Poulter enamored with acting even when so many other things about his chosen career can be tricky. “It can be stressful, it can be painful, and plainly speaking, it can be difficult to do and a strain on your mental health, but I also think it’s very necessary to reflect on your own psyche and think about its impact on the world around you,” Poulter said. “It’s a lovely psychoanalytical journey that I’m really enjoying.” More