Actors are directing. Singers are acting. Drama directors are making concert films. Former presidential rivals Al Gore and Ralph Nader are hitting the big screen.

And Hollywood's much-maligned system of rating movies stars in its own film.

The Sundance Film Festival, the country's foremost showcase for independent cinema, gets under way Thursday with an intriguing mix of role reversals among its cast.

Gore and Nader lead what's shaping up as a powerhouse year for documentaries, always a strong suit at Sundance. Director Davis Guggenheim's "An Inconvenient Truth" chronicles former Vice President Gore's dogged campaign to convince a reluctant society of fossil-fuel profiteers and consumers about the dangers of global warming.

Nader, viewed by critics as the spoiler whose campaign kept Gore out of the White House in the 2000 election, is the subject of Henriette Mantel and Stephen Skrovan's "An Unreasonable Man," a portrait of the crusader for consumer rights and safety.

Sundance opens with writer-director Nicole Holofcener's "Friends With Money," starring Jennifer Aniston as a woman in limbo about her future after quitting her job and taking up temporary work as a housecleaner. The film centers on her relationship with three affluent friends (Frances McDormand, Catherine Keener and Joan Cusack).

It's a return engagement for Holofcener, who premiered her feature-film debut "Walking and Talking" at Sundance in 1996 after developing the story in the Sundance Institute's writing and directing labs.

Back then, Holofcener was an unknown who made a splash at Sundance. How does she feel about kicking off the festival with a star-driven flick?

"Thrilled and petrified. Equal measures of both," Holofcener said. "Thrilled because I have a history with Sundance, which made this feel special and like a real honor. Petrified, I guess, because I think it's never good to go into a film with really high expectations. That can't be in my favor. That's not to say it won't meet them, but what if it doesn't? I hope people will have goodwill toward it."

Holofcener shouldn't worry. Sundance audiences, especially on opening night, are a receptive bunch, fired up by the prospect of 11 straight days of indie film.

This year's festival presents 120 feature-length films and dozens of shorts.

Actress Joey Lauren Adams ("Chasing Amy") directs Ashley Judd in "Come Early Morning," a drama about a Southern woman struggling to turn around her self-demeaning life. Comic Bob Goldthwait directs "Stay," a romantic comedy about a relationship strained to the breaking point by a pact of absolute honesty.

Jonathan Demme, the Academy Award-winning director of "The Silence of the Lambs," is showing "Neil Young: Heart of Gold," which captures the rocker in concert accompanied by Emmylou Harris in Nashville last year.

Singer Justin Timberlake joins Emile Hirsch, Bruce Willis and Sharon Stone in Nick Cassavetes' "Alpha Dog," a tragic tale of rivalry and violence among young drug peddlers. Musician Tom Waits is among the cast of "Wristcutters: A Love Story," Goran Dukic's offbeat film about a dreary afterlife reserved for people who have killed themselves.

An institution of the movie industry takes its knocks in Kirby Dick's "This Film Is Not Yet Rated," a study of the Motion Picture Association of America's system for rating films. The ratings board has come under frequent fire as overly prudish on sex and permissive on violence, with some critics and filmmakers likening it to a censorship panel.

"After watching what happened for 20 years with the ratings board and all the criticism from critics, filmmakers, even people around the country, and nothing changing at all, I felt it was really time to set out to make a film," said Dick, a 2004 Oscar nominee for his documentary "Twist of Faith."

"The most unfortunate thing about the system is the secrecy of the board. That was one of the things I wanted to break through with my film."

Among other Sundance highlights: Finn Taylor's "The Darwin Awards," with Winona Ryder and Joseph Fiennes in a twisted comedy about people accidentally killed in idiotic ways; Dito Montiel's "A Guide to Recognizing Your Saints," starring Robert Downey Jr. and Rosario Dawson in a mean-streets drama set in 1980s Queens; Laurie Collyer's "Sherrybaby," featuring Maggie Gyllenhaal as a prison parolee trying to rebuild her life; and Isabel Coixet's "The Secret Life of Words," with Tim Robbins and Sarah Polley in the story of a nurse tending a temporarily blinded man on an oil rig in the Irish Sea.

As Sundance has grown from its roots as Robert Redford's little place of discovery and nurturing for new talent, celebrity hoopla and corporate marketing gimmicks often have overshadowed the films. Critics say Sundance has gone commercial, yet defenders insist such trappings are outside festival organizers' control.

"There's a bunch of, for lack of a better description, carpetbaggers attending the festival," said Kevin Smith, who established himself with "Clerks" at Sundance in 1994. "This parasitic community that kind of attaches itself to the festival but has nothing to do with the festival. But the festival takes the knocks for it. ...

"I've always had an incredibly warm feeling about the place," said Smith, who returns to Sundance as executive producer of buddy Malcolm Ingram's documentary "small town gay bar," about oases that homosexuals find in the Deep South. "I went there as a dude with a job at a convenient store and left there as a dude with a film career. That place changed my life in a matter of about four days.