The supporting actor race is expected to be crowded. Jared Leto, Casey Affleck and Jake Gyllenhaal all underwent dramatic transformations to get into character and their work could mean an Oscar.

Staying in character requires commitment. Among this year’s crop of supporting actor Oscar hopefuls, Jake Gyllenhaal, for his role as a police detective hunting for missing children in “Prisoners,” covered his body with temporary tattoos. In the blue-collar drama “Out of the Furnace,” Casey Affleck got bruised in bare-knuckle boxing sessions. Bradley Cooper endured an unfortunate, but period-appropriate, perm in “American Hustle.” And Jared Leto lost more than 30 pounds and shaved off his eyebrows, among other make-unders, to play a transgender AIDS patient in “Dallas Buyers Club.”

Leading men in Oscar-bound films are often called upon to metamorphose, most often from hunkiness to debilitation, chiseled hero to lout (and back again). But actors in roles with sparser screen time may do even more, transforming themselves physically and walking off with their scenes with portrayals of depravity or verbal dexterity. (Christoph Waltz in “Inglourious Basterds” and “Django Unchained” had both.) The emotional turns, in shorter sequences, become more harrowing: Christian Bale embodied both the spirit and the failure of “The Fighter” in just one scene. And bad haircuts are surprisingly effective for prizewinners: Witness Javier Bardem’s shaggy pageboy in “No Country for Old Men.”

This Oscar season, supporting actor hopefuls are making all the usual moves — transforming bodily and emoting greedily — along with some unexpected shifts. There are comedians doing understated drama (and vice versa, as in James Gandolfini’s turn in the romantic comedy “Enough Said”) and foreign-born actors diving into American films in a major way. And at least one front-runner, Michael Fassbender, has publicly eschewed award campaigning for his performance as the sadistic plantation owner in “12 Years a Slave.”

Most pundits see Mr. Leto and Mr. Fassbender as the leaders in the supporting actor race; both have earned precursor awards, from industry groups and critics, and are part of films with widespread trophy momentum. But the category is often ripe for surprises.

Surging ahead are Daniel Brühl, a Spanish-born German actor playing the Formula One driver Niki Lauda in “Rush,” and Barkhad Abdi, a Somali actor from Minneapolis making his feature debut, opposite Tom Hanks, in “Captain Phillips.” Both Mr. Brühl and Mr. Abdi have gotten nominations from Golden Globes voters and the Screen Actors Guild, whose membership overlaps with the Academy’s. The Golden Globes winners will be announced on Sunday, and the Oscar nominees next Thursday.

Mr. Affleck and Mr. Gyllenhaal, though far better known, are longer shots, with neither having been nominated for the major precursor awards. “Out of the Furnace,” from the writer-director Scott Cooper (“Crazy Heart”), made barely a dent at the box office. But for Mr. Affleck, the story and the chance to work opposite Mr. Bale, who plays his brother, were the draws. What was important, Mr. Affleck said, was that his character, a down-and-out Iraq war veteran, “has this interior life that he can’t share it with his brother, he can’t share with anybody, and it’s eating him up.”

Boxing was the character’s outlet, and the challenge for Mr. Affleck. From YouTube videos of illegal matches he learned that the fights didn’t look perfect — “not at all like ‘Bourne Identity,’ ” he said. “So I had that going for me, because it was supposed to look sort of sloppy and messy and not very slick and in control. And I was not very in control.”

Mr. Gyllenhaal’s detective in “Prisoners” was similarly withdrawn; he had hardly any back story, so Mr. Gyllenhaal invented it, envisioning a past of institutions — children’s group homes and juvenile detention centers — that left an unwanted mark. “I wanted the tattoos, so that I could hide them, not so that I could show them off,” he said.

For Mr. Leto, “Dallas Buyers Club” is a showy return to acting after five years of focusing on his band, Thirty Seconds to Mars. “For a few years, the scripts were still coming, but after that, people get the point,” he said. But when “Dallas Buyers Club” — starring Matthew McConaughey as the real-life AIDS patient and experimental treatment crusader Ron Woodroof — came his way, he was ready.

He auditioned for the director, Jean-Marc Vallée, over Skype, before a gig in Berlin, playing the character, Rayon, even then. “I had a little pink sweater on and pulled that over my shoulder, and I proceeded to flirt with him for the next 20 minutes,” Mr. Leto said. “And woke up the next day with the offer.”

He stayed in character for the 25-day shoot. “Every morning, no matter what, I stepped out of that passenger van when I got dropped off on set, and I was wearing my heels,” he said.

Though he gained the weight back, leaving Rayon behind wasn’t easy. “She’s funny and fun and full of grace and wit,” Mr. Leto said.

For different reasons, Will Forte is not at all eager to move past his role in Alexander Payne’s black-and-white “Nebraska,” relishing the accolades he’s earning for playing against type.

“I am not used to getting awards,” he said at the National Board of Review gala on Tuesday, where he picked up a prize for his performance as a wayward son on a road trip with his father. “In fact, I am used to the opposite of getting awards. I am used to the ‘Why did you do that?’s. Or the ‘What the hell were you thinking?’s.”

Mr. Forte, a former cast member of “Saturday Night Live,” said he never expected to be in a film by an Oscar winner like Mr. Payne, and he was petrified that the cast, including veterans like Bruce Dern, had acting secrets that he did not. “When you watch this movie, you’re watching somebody who’s in the process of being taught by a legendary actor and an amazing director,” he said in a recent interview.

Mr. Hill was likewise attached to the experience of making “The Wolf of Wall Street,” though he was given full license to improvise, beginning with those in-character calls to Best Buy and Target. He also worked closely with the costume designer, Sandy Powell, to perfect the look of his character, Donnie — preppy sweaters around his shoulders to appear WASP-ier and ties whose garishness corresponds to his wealth. “So much of Donnie are his ridiculous outfits,” Mr. Hill said.

He was less worried about working with Mr. Scorsese than about hanging out with him — “I didn’t know if I would say something stupid, personally,” he said — but called the production a pivotal one in his career. “In my note I wrote him afterwards, I said, ‘After this, it’s just egg noodles and ketchup,’ ” Mr. Hill said. And he still has the teeth — at home, in a safe, as a souvenir.

Watch the full interview here: