"No holds barred," Richard Nixon urges to David Frost as the two prepare to sit down for a series of interviews in 1977.

As "Frost/Nixon" powerfully reveals, that statement contains equal parts promise and threat from both the disgraced figure on screen and the actor playing him.

Frank Langella is positively formidable as the former president, a skilled manipulator under optimal circumstances whose desperate desire for rehabilitation makes him extra dangerous.

Langella isn't doing a dead-on impression, which is preferable; Nixon's quirks have been imitated so frequently and poorly, such an approach risks lapsing into caricature. Rather, he has internalized a volatile combination of inferiority, awkwardness, quick wit and a hunger for power. He loses himself in the role with rumbles and growls, with a hunched carriage and the slightest lift of the eyebrows.

Langella and Michael Sheen, also excellent as the breezy British TV personality Frost, reprise the parts they originated in Peter Morgan's Tony Award-winning stage production but you never feel like you're watching a play on film. The way Morgan has opens up the proceedings in his screenplay feels organic under the direction of Ron Howard, who's crafted his finest film yet and one of the year's best.

"Frost/Nixon" is talky and weighty as it digs into the details of Vietnam and Watergate, but it moves along with a fluidity that keeps it constantly engaging. Morgan's script also contains a healthy amount of dark humour, mostly the result of something crass or inappropriate Nixon has said. Good thing, too, because the tension starts percolating early and only grows.

Upon seeing the image of Nixon smiling eerily as he boards a helicopter after resigning the presidency, Frost stands in front of a television transfixed. Hoping to lose the perception that he's a lightweight and gain some credibility -- or rather, achieve fame in America -- he approaches Nixon for an interview and promises money he doesn't have.

Sheen is doing something so subtle here and, as in his insightful work as Tony Blair in Morgan's "The Queen," he's likely to get upstaged, unfortunately. All his Frost wants is to be liked but he strives for that with the slightest obsequiousness. Critics may mistake his playboy demeanour for arrogance but it truly seems to spring from longing.

The former president, meanwhile, hopes to use the opportunity to return to public life among the East Coast elite. He's bored with retirement and feels humiliated droning on for banquet crowds for cash. He wants an interviewer with heft but he'll take the $600,000 his agent, Swifty Lazar (Toby Jones), has secured by saying yes to Frost.

And so they face each other for four extended interviews, which comprise the film's second hour. Frost has gotten help cramming for this exam from British TV producer John Birt (Matthew Macfadyen), veteran journalist Bob Zelnick (Oliver Platt) and author and Nixon critic James Reston Jr. (Sam Rockwell). Rebecca Hall provides moral support as the sultry socialite Frost picked up in first class while flying to the United States.

In Nixon's corner are loyalists including the fierce strategist Jack Brennan (Kevin Bacon) and, amusingly, a young Diane Sawyer. Performances from the chief supporting players are uniformly excellent, especially from Platt and Rockwell, whose characters rib each other and share a disdain of Frost's celebrity.

But Zelnick puts it best when he calls Frost "the most unlikely white knight . . . but a man who had one big advantage over all of us. He understood television." And television exposed both Frost and Nixon for their true natures -- for better and for worse.

Four stars out of four.