Acting depends, to a degree, on the playwright's material, though skilled acting by a charismatic personality can, and often does, disguise flimsy lines. Star players, men and women, have devoted years to shoddy plays - and their memoirs reveal their regrets.

The ideal play has many roles that can be acted many ways - the cast of "Hamlet" being an example - and the ideal of every performance is of a wholly realized ensemble.

One trouble is that playwrights don't write that way. Our attentions are pressed onto limited characters and on these central figures a performance must rise or fall. There are star parts and star performers.

Whatever one may think of this season's offerings, a few have displayed exceptional acting by star performers, among them Julie Harris in "The Belle of Amherst" and the Gielgud-Richardson team of "No Man's Land," which, with its two lesser characters, did achieve an ensemble effect.

Now the Eisenhower boasts a star part for a star actor, and in Tom Stoppard's "Travesties" John Wood is turning in a brilliant "star" performance.

Consider his Henry Carr. We first meet him as an old man in slippers, a robe and hat. Small, beady eyes peer out at us, the slippers shuffle him about and a breathy voice further suggests age. During his monologue, gradually yet swiftly, Carr eases out of his slippers, shakes his thin frame out of his robe, removes the hat, brings his voice down some notches and, voila, there is young Henry.

There is more to this than the technical trick of revealing a flashy, youth-oriented suit. Wood's long, angular frame is a help, so is his face, as is his voice. But the triump lies in the actor's total assurance of what he is doing: just enough, no more, no less.

Carr is mercurial, a fast-talker. Time-shifts include what he prefers to think happened, and what happened, more or less as he remembers it. That he conveys all this minute precisely makes the viewer gasp.

Granted, the part is exceptional. But Wood is equal to it, master of it, glories in it, infuses his watchers with his own delight.

For a star actor such as Rex Harrison, however, a dazzling part can become a curse. His Prof. Higgins of "My Fair Lady" was so decisive, so considered, so balanced, so complete, that Harrison can be said to have had in it the role of a lifetime. Though he limited himself, he did play it for several years and made the film known to millions.

Harrison had aplomb from the start.

On his American bow he neatly stole a little comedy called "Sweet Aloes" from its star, Evelyn Laye. That first American night for Harrison was here at the National Theater in the '30s. He was arresting, suave, attractive. He followed it up not only with screen success but deft, deceptively easy stage work, from "The Love of Four Colonels" and "Bell, Book and Candle" to Maxwell Anderson's Henry VIII in "Anne of the Thousand Days."

While he has acted both Shaw ("Major Barbara") and Caesar (the film "Cleopatra"). Shaw's Caesar is new to him and so far on the Opera House stage, eludes him. While this is a comedy and Caesar does have all the good lines, the conqueror's inner iron, at the age he was meeting Cleopatra, would be more suitable than the steel here bought to it. Harrison's Caesar, inclined to forsee age and [WORD ILLEGIBLE] , does not express the soldier-ruler. To a degree, too, the performance suffers from Harrison's public image, the star actor as public personality.

To a lesser extent, Elizabeth Ashley suffers from the same conflict. Her public personality, though not so clear as Harrison's, does get in the way of her Cleopatra. The part is less than Caesar's and those who happned to have seen, on either stage or screen. Vivien Leigh's Shavian Cleopatra must recall her feline delicacy. I've seen a peck of Shaw's Cleopatra and the only one comparable to Leigh's was Lilli Palmer's opposite Cedric Hardwicke. Ths was not a girl, as Leigh's suggested, but a devilishly fascinating, elusive full-grown cat. Avoiding the delicate touch, Ashley shows her as a tiger, assertive, abrasive.

Another star part - again, an English actor in a highly literate English comedy - is Tom Courtenay's in the National's "Otherwise Engaged." But it is far harder, less rewarding a role than Carr or Caesar, less showy, more passive.

Throughout the play Courtenay's [WORD ILLEGIBLE] is acted upon. He is being kept from playing his new "Parsifal" recording by family and friends who persist in smashing his solitutde. He has to listen to their troubles. Courtenay is saying to us: Have you ever spent a whole day listening to other people's troubles? The listening begins so patiently, smilingly. This, Simon thinks, is bound to end and I can get back to Wagner. Smiles and murmers. Gradually the insides rebel. Eyes glaze. Fingers tighten. There may be a quick, bellowed reaction, as quickly controlled. Inevitably the repressions will shout their way to the surface.

Known for active roles in "The Loneliness of the Long Distance Runner" and "Charley's Aunt," Courtenay had romantic screen success in "Dr. Zhivago," but most of his recent years have been devoted to putting a major theater on the map in Manchester, the equivalent of doing what Marlon Brando said he would do but never did for regional theater. Older now, but in high regard of his acting peer, Courtenay extends his stage career to this treacherous star debut in America.

To its credit - and director Harold Pinter's - the "Otherwise Engaged" cast, prepping for New York, is achieving that skein of ensemble effect for which Pinter so carefully aims. While writer Simon Gray's scenes generally focus on two people, the effect of the performance is of a unity, the ensemble disiderata of theory. Neither "Travesties" nor "Caesar and Cleopatra" conveys this seamless effect.

How this can work, when it does, is exemplified to an enormously satisfying degree at the Kreeger, where four unfamiliar young actors, Terry O'Quinn, Joel Coldner, Brent Jennings and Damien leake, join two Arena Stage veterans, Howard Witt and Robert Prosky, in David Rabe's "Streamers." There are no stars there nor is there one single overshadowing character, but the totality realizes producer Zelda Fichandler's ambitions for ensemble acting.

In the three current British plays you get stars in star material. In the Kreeger's American work you find ensemble work at is best. There is a performance tonight at 7:30 and the British stars would find the ensemble style illuminating. So strikingly juxtaposed, these two styles of acting constitute a lesson in the art.