It's hard to think of another actor who can so effectively convince laymen of the joys of his profession as Kenneth Branagh. He waxes lyrical about acting and actors, the process of putting on a show or rehearsing a film. There's a boyish enthusiasm about him when he talks this way.

Here's an actor, it seems, who never encountered a role he didn't like. Until now, that is. In the chilling new HBO drama Conspiracy," which airs Saturday night at 9, Branagh plays Reinhard Heydrich, one of Hitler's most trusted lieutenants in the Third Reich; it was his job to set in motion the Final Solution, the extermination of millions of Jews across Europe in World War II. To effect these plans, Heydrich convened a top-secret meeting of 14 high-ranking Nazi officers in a mansion at Wannsee, in Berlin's suburbs, in January 1942. "Conspiracy" is a dramatization of that meeting. Some officers around the table are uneasy when mass extermination enters the agenda, and voice objections. But as the meeting proceeds, it becomes clear that the Final Solution is not up for discussion. It is a policy approved at the highest level, and the task of the eerily persuasive Heydrich is to find a consensus among the group about its implementation.

"I found it disturbing to portray the man," confided Branagh, over afternoon tea at a large central London hotel. "There's a spiritual revulsion against playing him. You don't want to be saying the things he was saying, or be part of his psyche. I found it got under the skin in an invasive way."

Still, Branagh plays Heydrich with verve. His hair dyed blond and swept back sleekly, he is the last person to arrive at Wannsee and makes a flamboyant entrance, immediately demonstrating his superiority. He goes on to run the meeting like the chairman of a corporation, sometimes showing deference and courtesy to other points of view, and frequently calling breaks for drinks and lunch to defuse tension, but ruthlessly proceeding toward a point where his 14 colleagues agree to genocide.

Most of the Nazi officers who attended Wannsee were obscure names--the exception being Adolf Eichmann, who was tried and executed for his war crimes in Jerusalem in 1961. He is played in "Conspiracy" by the cast's one American actor, Stanley Tucci; most of the others (including Colin Firth as an uneasy Dr. Wilhelm Stuckart, state secretary of the interior) are stage-trained British actors.

Director Frank Pierson placed most of the action of Conspiracy inside the meeting room--which was reconstructed at Shepperton Studios, near London, with four walls, to the exact dimensions of the original. (Exterior scenes at the beginning and end were shot at Wannsee.)

With several extended scenes and long tracking shots, it feels like a play being filmed. The eye-level camera angles underline the feeling for viewers of being present in that room. After rehearsing, it was shot in 21 days.

"I think the experience got under the skins of everyone involved in it," Branagh recalled. "You'd rehearse moments, and a piece of dialogue would hit home. We partly coped with it all through a lunatic Monty Python humor. When I read the script, my reaction was jaw-dropping astonishment at the tone of this meeting and the apparently easy, casual quality to the discussion of the fate of an entire race across Europe. Yet it felt like the quiet political infighting of a board meeting at a big company."

"Being around that casual tone, and the manner in which the material was dealt with was as shocking as one's exposure to more obviously horrific elements of the Holocaust itself. This was a job, farmed out by a Führer who had decided issues like this could be delegated. It was all about logistics, and those people 'round the table were irritated. It was an annoyance, all that administration being brought in to solve this Jewish problem. 'Oh, it had to be done, but what an annoyance.' That was the attitude."

Because of this contrast between the exasperated, morally indifferent manner in which these Nazis contemplated the Final Solution, and the unimaginable awfulness of its consequences, Pierson and his cast chose to play down the melodrama inherent in the Wannsee meeting.

Conspiracy screenwriter Loring Mandel said Pierson and editor Peter Zinner became interested in doing the movie after they saw a subtitled 1984 Austrian-German docudrama, "The Wannsee Conference." Initial research revealed that minutes of the conference contained no direct quotes. The filmmakers then did extensive independent research on the meeting and the background of all the participants. Loring said he turned in his first draft in the fall of 1996. Several years and many redrafts later, "Conspiracy" flirts with being undramatic for much of its 87-minute length. As the Nazi officers arrive at Wannsee to be greeted by Eichmann, much is made of their repetitive "Heil Hitler!" salutes--to the extent they become creepily amusing. The camera lingers over the food and drink consumed at the meeting, and the place settings; Hannah Arendt's memorable phrase about "the banality of evil" often comes to mind.

"The idea was to stay away from being theatrical, and resist the lure of easy melodrama," said Branagh. "There was no desire to catch great moments. It's obvious from the way it was written that some of the information deserved to ring on the air a bit, but Frank tried to take that out."

Another problem for Branagh was finding anything in Heydrich's character or upbringing that might explain his cold-blooded willingness to undertake genocide on such a horrific scale: "But nothing in his background supplied any clues. He had a loving, supportive family. There seemed to be no traumatic incidents in childhood, no sibling rivalry. In some ways he was an ideal Nazi--he was an excellent musician, an Olympian fencer. "One of the questions you have to try and answer is some definition that allows you to play the character of the man you're playing. But I discussed this with Stanley Tucci, and he felt the same about Eichmann as I felt about Heydrich. You feel there's nothing there.

"There was no compassion inside Heydrich. He had dirt on all fellow Nazis. Hitler and Himmler knew he was a lethal weapon who was happy to do all the dirty jobs. Anything no one else wanted to do, delving into moral backwaters, he had no problem with. Playing him, I felt if he had been asked to eradicate Eskimos, cabinet-makers or gymnasts he would have proceeded with the task in the same way, with the same passionless, soulless quality."

For all his personal reservations about playing a character such as Heydrich, Branagh is happy to have been a part of "Conspiracy." "I thought it was an important story to be told," he said. He was also impressed by the attitude of HBO Films, a company he believes is now tackling substantial stories that might have once found a natural home at major movie studios. "It's hard to imagine [Conspiracy] being financed in a feature context," he noted, "or for it to have been cast with the kind of actors we had. It was not about trying to be starry or grab attention.

"There's an audience for these kinds of stories, certain kinds of serious, not solemn films. HBO has found a creative identity, which is drawing filmmakers and actors because of the freedom it offers and the originality of the material."