Jack Dempsey, the charismatic heavyweight champion of the Jazz Age, spent a good portion of the 1920s partying, endorsing products, giving the occasional exhibition and even making movies. What he didn’t do for three straight years was defend his crown. When he did, in 1926, he lost.

Julia Roberts doesn’t look anything like Jack Dempsey, but she is the reigning heavyweight champion — the $20 million hire, the female star who can open a movie with one arm tied behind her back. But for the last three years, she’s given the thesp version of exhibitions (vocal performances in the animated “Charlotte’s Web” and “The Ant Bully”) and reprised a relatively minor role, as Tess in “Ocean’s 12.” She’s also managed to star in a Broadway play and have three children, something Dempsey never did. But movie-wise it’s been a bit of a layoff since her ensemble drama, “Closer,” in 2004, and her last real starring vehicle, “Mona Lisa Smile,” in 2003. On the occasion of her American Cinematheque award Oct 12, one wonders whether the Siren of Smyrna, Ga., can get back in the ring and slug it out with the girls.

Like Billy “Piano Man” Joel and California’s “Guvernator,” Arnold Schwarzenegger, Roberts was fortunate enough to have an early success that also provided a trademark: “Pretty Woman.” Helmer Garry Marshall’s fatuous fairy tale may have promoted a thoroughly unrealistic notion of prostitution, but the politics were obliterated by Robert’s radiance and a laugh like the pop of a champagne bottle. The signature Roberts eruptions — think of her lighting up the screen in “Mystic Pizza,” as the giggling Tinkerbell of “Hook” and even, intermittently, in such half-measures as “My Best Friend’s Wedding” — became a source of spontaneous, infectious joy.

Robert’s next release will be “Charlie Wilson’s War,” co-starring Tom Hanks and based on the book by the real-life title character about his covert operations in Afghanistan. It’s just one more example of how Roberts, for all her uber-celebrity, has consistently appeared in movies with socially progressive themes — from the domestic-abuse motif of “Sleeping With the Enemy” to her Oscar-winning eco-centrism of “Erin Brockovich” to the proto-feminism of “Mona Lisa Smile.” Her more adventurous work — with Steven Soderbergh, for instance, in such Soderberghian digressions as “Full Frontal” — indicate an impressive intelligence, if not the capacity for full immersion in a given role. Or, maybe, a preference for roles that don’t require full immersion.

An Oscar-winning actress is destined, like it or not, to be defined by that Oscar-winning role. Anyone who’s made that much money for the film industry is already a good bet. For Roberts, it was “Brockovich,” a movie that followed the Academy truism that there’s no surer way to a statuette than abandoning one’s pigeonhole — such as being the pretty face in a series of thrillers (“The Pelican Brief,” “Conspiracy Theory,” “I Love Trouble”) and romantic comedies (“Something to Talk About,” “Notting Hill,” “Runaway Bride”). If there’s been a sure thing in the history of Oscars, it was Roberts’ actress award.

In her professional life, Roberts’ most rewarding quality may be her generosity — she’s shown herself willing to share the screen, even take subordinate roles, and done some of her better work in the process. “America’s Sweethearts” wasn’t a masterpiece, but as Kiki, Catherine Zeta-Jones’ sister and assistant, she was not the center of attention, and shone. In “Mona Lisa Smile,” she played the mentor to three of the hottest actresses of the next generation — Julia Stiles, Kirsten Dunst, Maggie Gyllenhaal — and underplayed. In one of her more endearing moments, she appears in a supporting role in the grossly underrated “Confessions of a Dangerous Mind” and is enchantingly unguarded and sassy, perhaps because she’s being directed by her friend George Clooney, or perhaps because she’s not the focus of the white-hot light that shines on Julia Roberts like it shines on so few others.

As with Bruce Willis, Bill Murray, Brad Pitt and several other megawattage movie stars who have occasionally appeared in nonstarring roles, Roberts seems most content in the margins, able to practice her art without being in the center ring. This doesn’t mean she doesn’t steal all the attention. But there’s some satisfaction for us all in her stealing it.