There’s nothing critics, audiences and Oscar voters love more than transformation. Put a prosthetic nose on Nicole Kidman (“The Hours”) and she finally wins an Academy Award. Ugly up Charlize Theron (“Monster”) and she’s suddenly kudos-worthy. But as much as everyone enjoys watching people play against type, there is someone who appreciates it even more — the actor.

Performers thrive on shape-shifting, and cherish the opportunity to unexpectedly delve into characters outside their usual comfort zone.

To that end, 2014 was a year marked by several such notable reinventions.

Even an Oscar winner like Reese Witherspoon can still surprise us. Largely known for playing strong, sunny characters, Witherspoon did a 180 to portray real-life author Cheryl Strayed, a recovering heroin addict who leaves her life behind to take a physical and spiritual journey. In “Wild,” she not only bared her body but her soul, and delivered a revelatory performance that has resonated with festival audiences, ahead of the film’s Dec. 5 bow.

We’ve also come to expect great things from Julianne Moore, but even her most ardent fans figure to see another side of her in “Still Alice,” in which she plays a professor diagnosed with Alzheimer’s. “Alice” also debuts Dec. 5. And while much has been made of Jennifer Aniston going without makeup as a woman suffering from chronic pain in “Cake,” the real talk should center on her daring, raw performance — a portrayal that more than one critic has called her “Monster.”

Eddie Redmayne’s awards heat index is practically fissionable for his startling turn as Stephen Hawking in “The Theory of Everything,” in which the actor transforms before our eyes as the renowned physicist who’s stricken with ALS. Likewise, his co-star, Felicity Jones, should finally ascend to the A-list with her heartbreaking performance as Hawking’s wife, Jane.

Meanwhile, Jake Gyllenhaal and Steve Carell took on the darkest roles of their careers. Gyllenhaal shed some 30 pounds to play an unscrupulous videographer in “Nightcrawler,” and Carell underwent three hours of makeup daily to turn himself into murderous billionaire John du Pont for “Foxcatcher.” But these performances don’t rely on just weight loss and prosthetics. Gyllenhaal is a live wire, pulsing with intensity as a terrible human being you can’t help but be drawn to. And Carell is a revelation, filling every scene with impending dread while bringing full dimensionality to an unsettling man.

And while weight gain or loss is a way to get attention, the performance has to back it up. Nobody cared when Jared Leto gained more than 60 pounds and gave himself gout to play Mark David Chapman in “Chapter 27,” because the movie was dismissed. But when he lost weight for “Dallas Buyers Club” to startlingly play an AIDS-stricken transgender woman, a best supporting actor Oscar came his way. Reviewers have noticed Bradley Cooper packing on 40 pounds and adopting a Texas drawl to limn Navy SEAL Chris Kyle in “American Sniper” (due to hit theaters Dec. 25) because the performance delivers.

Some actors were welcomed back to the bigscreen after too long an absence in major roles. Is there any other thesp who could have played Riggan Thomson in “Birdman” than Michael Keaton, an actor famous for portraying a superhero who longs to be taken seriously in a new role? But beyond the meta in-joke, Keaton brings years of experience, top acting skills and a whole lot of goodwill to the role.

Also making a comeback, Patricia Arquette, who made her mark 21 years ago with “True Romance,” was the beating heart of “Boyhood.” In fact, many actors who made a splash at a young age, such as her “Boyhood” co-star Ethan Hawke, and Laura Dern of “The Fault in Our Stars” and “Wild,” are now convincingly playing parents.

It was also a great year for breakthroughs — an act of transformation in its own right. Everyone knows J.K. Simmons’ face as the insurance pitchman, “Law & Order” veteran and Spider-Man-hating newspaper editor, but no one will be able to forget his name after seeing how he terrifies audiences in “Whiplash.” Actors who have done solid work for years but finally landed star-making roles include David Oyelowo as Martin Luther King Jr. in “Selma” and Benedict Cumberbatch as code-breaker Alan Turing in “The Imitation Game.” And fresh new faces caught our eye, including Gugu Mbatha-Raw (“Belle”) and Ellar Coltrane (“Boyhood”), with Jack O’Connell waiting in the wings in “Unbroken” (debuting Dec. 25).

Of course, there are actors who don’t seem to ever struggle with typecasting — Jessica Chastain appears in three wildly different roles, going from grieving mother (“The Disappearance of Eleanor Rigby”) to a loving, brilliant daughter (“Interstellar”) to a tough wife with mob connections (“A Most Violent Year”), morphing into a new incarnation seemingly every month.

Ultimately, in a year so given to transformative roles, who better to engage in a dialogue about all those changes, the stories behind them and the work that went into them than the actors themselves?

For our Actors on Actors conversations, be sure to view our videos at variety.com/contenders.