As the reign of a once popular but now unpopular wartime president draws to a close, it feels timely to look back at another such commander-in-chief, one Richard Milhous Nixon. Watergate has been done to death in the movies, but “Frost/Nixon” is a superbly enjoyable examination of one of its postscripts: the 1977 conversations between the erstwhile president and a lightweight TV interviewer named David Frost. Thanks to an executive pardon, this was the closest thing to a trial Nixon would ever face, and the dramatization of it is electrifying.

It’s based on a successful play by Peter Morgan, who adapted it for the screen after penning the similarly fact-based political dramas “The Queen” and “The Last King of Scotland.” The director is Ron Howard, whose workmanlike approach — he tends to set the stage, then let the actors do their thing — serves him well here, where a flashier style would only get in the way.

Nixon is played by Frank Langella, with the Frost role taken by Michael Sheen, who played Tony Blair in “The Queen.” That’s the same configuration as the original London production of the play, as well as the Broadway version, for which Langella won a Tony. In other words, these two have played these characters at least 300 times. Can it still have any vitality after so much repetition? To misquote another president, yes it can!

The bulk of the film actually focuses not on the interviews but on the preparation for them. Frost, a British TV host with his fingers in several different entertainment pies, is eager to make a dent in the American market after his first stab at a U.S. talk show proved unsuccessful, and he fixates on the idea of interviewing Nixon after seeing how many hundreds of millions of people worldwide watched his resignation speech in August 1974. Surely a one-on-one with the disgraced ex-president would be just the thing Frost needs to jump-start his Stateside career.

The problems are manifold. For one, Nixon isn’t exactly doing a press tour these days, and he certainly doesn’t want to discuss Watergate. For another, no one takes Frost seriously as a political journalist. He’s more Ryan Seacrest than Edward R. Murrow, and his cavalier attitude toward preparation once Nixon signs on only reinforces his associates’ misgivings.

Those associates include his producer, John Birt (Matthew Macfadyen), whose primary concern isn’t that Frost’s approach will be shallow but that they won’t be able to find an American network to air it. A post-resignation Nixon interview sounds like a no-brainer, but the broadcasters all fear it will be a debacle, like having Carson Daly interview Kim Jong Il. Nobody wants to foot the bill for a train wreck (or at least they didn’t in the ’70s; they all seem eager enough to do it now).

Frost also has a pair of researchers on board to help him prepare for the four two-hour interviews. James Reston (Sam Rockwell) is an ardent Nixon hater who sees this as a shot at vengeance against his mortal enemy; Bob Zelnick (Oliver Platt) is somewhat more pragmatic though still a dogged investigator. In Nixon’s corner, he has his press agent, the gloriously named Swifty Lazar (Toby Jones), who convinces the ex-prez to do the Frost interviews the old-fashioned way: by getting him a $600,000 paycheck. Nixon is also aided by his chief of staff, Jack Brennan (Kevin Bacon), an angry strategist who refers to liberals as “hippies, draft-dodgers, and dilettantes.”

If it sounds like a high-profile boxing match, with weeks of preparation leading up to the main event, that’s exactly right. That metaphor is even clearer when the actual interviews begin, as the trainers hiss advice to their fighters during breaks and urge them to follow the right strategies to bring their opponents down. Nixon is a scrapper who loves a challenge, while Frost is the underdog who seems mismatched against such a formidable foe.

Morgan’s screenplay and the outstanding lead performances manage to create suspense even when we already know the outcome. It’s not a foregone conclusion that Frost will wrangle a confession out of Nixon — who, don’t forget, is not on trial and is free to walk out anytime. Frost has to be cagey, and he has to overcome Nixon’s knack for manipulating conversations.

Sheen plays Frost perfectly as a man whose ambitions may or may not outrun his abilities, and while Nixon is ultimately the focus, I marvel at how Morgan establishes commonality between the two men. Both will stop at almost nothing to achieve success in America; one is still striving for it while the other has suffered the consequences of striving too hard. There is a healthy respect between Frost and Nixon, and between Sheen and Langella.

There’s no question Langella is the heavyweight, though. His impersonation of Nixon’s voice and mannerisms is, you’ll excuse the term, unimpeachable, but it goes beyond that, well into the realm of legendary performances that will stand the test of time. It’s safe to say no one will ever play Nixon as effectively as Langella does here, with all the nuance and subtlety befitting one of the 20th century’s most important leaders.

In the climactic moments, Langella takes advantage of what cinema can offer that Broadway theaters can’t — the intimacy of the close-up — to present Nixon as a tragic, sad, and remorseful figure. Who would have thought we’d come away feeling so touched by Nixon’s humanity? It’s not that we feel bad for him; he’s only lying in the bed of his own making, after all. It’s that we see him not as a hunchbacked caricature but as a flesh-and-blood human being who fell from grace the way anyone could.

B+ (2 hrs., 2 min.; R, 4 F-bombs, a brief glimpse of partial nudity.)