Jason Robards is certainly the star, technically and otherwise, of "The Iceman Cometh." But in a cast of wonderful actors, Donald Moffat is a standout, and his portrayal of Larry Slade, the one-time anarchist who has decided to live his life in "the grandstand," ranks as essential viewing for anyone seriously interested in the craft of acting.

Moffat, 54, left England nearly 30 years ago and has accumulated an arm's length of credits since then, including work at both major and minor resident theaters, Broadway and off-Broadway. He got a Tony nomination for his role in "Right You Are . . . If You Think You Are," and an Obie for "Painting Churches." He is quite frank about supporting his theater "habit" with movie and television work, including such films as "The Terminal Man," "The Thing," "Popeye" and "The Great Northfield, Minnesota, Raid." Last year he was seen as the crusty and ailing father in "Alamo Bay," and he was Lyndon Johnson in "The Right Stuff." He is one of those actors who seem familiar although you don't know why. He was last in Washington in a touring company of "Listen to the Mockingbird" in the late '50s, which had the distinction of being in residence at the then-Shubert theater when it burned down.

He went after the part of Slade when he first heard of the proposed revival. "It's an amazing part," he said. "The piece is so musical -- you need the strong note to sound against, and he's the one. He is the only character in the play who has given up his pipe dream at the end of the play. Even Hickey Robards has begun building one again. But Larry is faced with fact; he can't pretend to be in the grandstand anymore." He is also one of two characters who are on stage during the entire play, and the only one who never sleeps.

Rehearsals, he said, were "painful and long . . . the driving force is the psychological need to meddle or lash out. Underneath there are really ugly and painful things to explore . . . You dig and dig and dig, and the tendency is to substitute some skill you as an actor are good at. So there's a long period when you walk around feeling totally inadequate -- that it will never jell."

Director Jose Quintero, with whom Moffat had long wanted to work, did not give notes at the end of rehearsals. "He left us to our own resources. Sometimes he would stop someone from doing something. What he would do is come up on stage and take the actor's position, and have you feed him the lines while he demonstrated in body English -- or body Spanish -- what he wanted. Some of the actors found that hard to deal with at first."

Some of the difficulties facing the cast were purely technical. During the first hour, for example, nobody moves from his seat in Harry's Bar. Most of the characters, in fact, are asleep, their heads on a table or resting on a hand. Also, the length of the play made it difficult to have a run-through. "It could take all day," Moffat said. Incidentally, Eisenhower Theater manager Max Woodward says the five-hour running length does not seem to have discouraged customers, since the show is selling well and there have been few walkouts. The line at the bar, however, usually is long after the first act.

The character of Larry Slade is based on a man named Terry Carlin, whom Eugene O'Neill met during his low-life bar days and who introduced him to Nietzsche, Strindberg and nihilist philosophers. Moffat used for his observations some of the people profiled in "Seeing Red," a documentary about American Communists.

"I panicked at one point, thinking I would never be able to make it convincing. I couldn't get the words to come through me. Finally you just give yourself over to his constructions and stop trying to figure it out . . . The other night the third act was off, and later Jason said he knew we had lost tension as soon as he came on. 'Somebody must have been acting out there,' he said."

Moffat added: "An intellectual approach to this play is absolute death."