At the end of every episode of “Inside the Actor’s Studio,” host James Lipton asks his guest the “Bernard Pivot questionnaire.” Lipton’s infamous questions on career choices, Academy Awards and inspiration suddenly become, “What turns you on?” and, “What is your favorite curse word?” It’s a highlight of the program: moving away from film-as-craft and putting the high-profile star at ease, which, in turn, incites only the best and sometimes the most inappropriate of responses.

Moments like this provide a unique glimpse into the personalities of each guest, and more specifically, how they approach acting. These interviews reveal that the great actors are not the celebrities who serve as daily tabloid fodder, but iconic figures who recognize their talent and enjoy the challenge of completely transforming into a character onscreen or onstage.

Some actors thoroughly prepare for each role, reciting their lines upwards of 250 times, while others rely more on improvisation and spontaneity, as Robert DeNiro and Jodie Foster ’85 did so well in “Taxi Driver.” What always remains true is that acting is a means of self-discovery. Actors do not have all the answers, just as we, the audience, have to discover for ourselves what separates a regular part in a movie from a truly masterful performance.

But what makes a great performance? When we think of the greatest roles in the history of cinema, certain names and films come to mind: Marlon Brando in “On the Waterfront,” Peter O’Toole in “Lawrence of Arabia,” Meryl Streep DRA ’74 in “Sophie’s Choice.” Why do they stand out? In some cases, it’s the preparation, physically morphing into character through weight loss or gain, deglamorizing and training months before filming. In others, it has more to do with presence. Despite the fact Anthony Hopkins is onscreen for only 16 minutes in “The Silence of the Lambs,” his chilling persona not only dominates the movie, but has you scared of moths and “fava beans and a nice Chianti” for the rest of your life (cue slurping noise).

As Feb. 27 draws closer by the day, the Academy will crown its favorites in acting. Although an Oscar win does not necessarily equal a great performance, it adds credibility to the work and gives the distinguished title of Oscar winner to the actor. Two of the odds-on favorites — Colin Firth in “The King’s Speech” and Natalie Portman in “Black Swan” — gave performances that not only represent high points in their careers, but also rank among the best in recent years.

What is so striking about Colin Firth in “The King’s Speech” is not how effortlessly he pulls off King George VI’s stammer or his relationships with his speech therapist Lionel Logue (Geoffrey Rush) and wife (Helena Bonham Carter). It’s how convincing he is as a royal figure, who transcends his status to become an everyday man trying to overcome a great obstacle.

Firth is a huge reason why “The King’s Speech” succeeds as a film. We sense his every fear, every emotion, from the first trip he makes to Geoffrey Rush’s door to the memorable F-bombs scene to the long walk to deliver his speech to a waiting nation. It is all there in Firth’s expressions, his charisma, his presence — and it is the mark of truly magnificent work.

Natalie Portman’s Nina Sayers in “Black Swan” could be seen as the subtler of the two performances, and the product of years of training. “Black Swan” makes it as easy to forget Portman’s track record (especially the “Star Wars” movies) as it was for her character to forget her mom’s presence in the bedroom — you know the scene. She nails the ballet, bringing emotion and physicality to Nina’s story. As she transforms from the innocent, unassuming white swan into the black swan, we see Portman bring about Nina’s descent into madness, becoming a victim to art and losing her grasp on the world around her. Her distress, frustration and ultimate satisfaction all coalescing in the final act mark the completion of her character arch and the great range in Portman’s performance.

Firth and Portman’s roles this year represented the cream of the crop, the best acting of the year, and they are sure to be honored as Best Actor and Best Actress at the Academy Awards. They carried their films and removed their sense of self to fully immerse in their characters. When answering the question, “What makes a great performance?” you need look no further than here.