So, just what did the butler do?

You don’t need to know or care who Doris Duke was, or the part her last butler, Bernard Lafferty, played in her life, to get something good out of “Bernard and Doris,” an excellent new HBO movie about their relationship. But I will tell you anyway: Duke was a tobacco and energy heiress who inherited her father’s fortune in 1925 at the age of 12, becoming a figure of public fascination in her time, much as Paris Hilton is in ours but without the sex tapes or reality show or trip to jail. At least in retrospect, she is often regarded as eccentric and lonely, but she doesn’t seem to have lacked for interests: She was a surfer, belly dancer, collector of Islamic art, animal lover, jazz pianist, foreign correspondent, orchid raiser and lifelong philanthropist. Lafferty, an uneducated, alcoholic, gay Irishman who had worked for Peggy Lee and Elizabeth Taylor, was the person to whom she left control of her own fortune when she died at 80, a circumstance some thought highly suspicious.

“Some of the following is based on fact,” a title card tells us as the film begins. “Some of it is not.” This, of course, can be said of every fact-based movie ever made. But it provides a preemptive defense against its departures from the record at the same time it gives the film a certain amount of traction. People love a “true story,” even if it isn’t all true.

But this movie is not exactly concerned with fact. It isn’t a full-fledged biopic, like the 1999 CBS miniseries “Too Rich: The Secret Life of Doris Duke,” with Lauren Bacall and Richard Chamberlain in the roles that Susan Sarandon and Ralph Fiennes splendidly take here. It’s a kind of pas de deux, rather, a character study of two people who become, in different ways and with different sorts of power, each other’s project. (I almost wrote, “two broken people,” but the picture is not sentimental like that -- if anything, it reminded me of the Mick Jagger film “Performance,” especially when Lafferty dresses in Duke’s clothes.) The point isn’t to understand what might have been going on inside the heads of the historical Duke and Lafferty -- because, you know, who cares? And who would know? It’s to use them as a pretext to explore the dynamics of an unusual relationship. (That is the difference between art and analysis, I suppose.) Apart from a scattering of servants and the odd lawyer, dinner guest, boy toy, cop or Indian holy man, they are the whole show: Everything interesting here happens between the two stars.

Credit is naturally due writer Hugh Costello for providing the words they speak. The dialogue does not aim for quotability but for natural rhythm and flow, a throwaway quality. You never feel that the speeches are straining to lay out the facts; rather, facts are used like seasoning, to flavor the drama. When Duke alludes to the daughter that lived for only a day, or Lafferty speaks of his mother’s accidental death, it is not by way of information but to change the temperature of the scene. Director Bob Balaban (an actor himself) knows where to point his camera and when to cut and keeps extraneous business out of the players’ way. His film speeds along, one might say, at a measured pace.

Sarandon is about 15 years too young for Duke when we first meet her and 20 years too young when we say goodbye, and no effort has been expended to make her appear any older than she is. At 61, she remains one of film’s most beautiful women, and when Duke is at last required to be sick, she still looks pretty terrific -- her hair goes lank, the makeup comes off, and the camera studies the relatively few lines on her face, but we should all look so good on our deathbed. (The bags under her eyes are strictly Louis Vuitton.) The 30-some years that separated Duke and Lafferty are twice what separates Sarandon from Fiennes; but age is never mentioned here, which among other things makes Duke’s sexual energy and overall physicality more plausible. Not that there aren’t sexually energetic septuagenarians, but on screen that might look like humor, or her age become too much the point.

Fiennes has the more contained role, but he does some sweet, subtle things with it. Sarandon is just remarkable. She never plays a single attitude -- there are always contradictory, complementary things going on in her performance, small, fleeting gestures or shifts in tone she makes legible without ever making obvious. She sings too. And very prettily.

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robert.lloyd@latimes.com

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‘Bernard and Doris’

Where: HBO

When: 8 to 9:45 p.m. Saturday

Rating: TV-14 (may be unsuitable for children under the age of 14)