CHRISTOPHER PLUMMER as Iago, Zoe Caldwell as Medea. In an otherwise depressingly flat theater season, two brilliant interpreters have stepped into two great roles.

The great parts--the big characters--are the measuring rods of acting. They are loaded with choices for imaginative actors. It is almost a crime that Orson Welles has done relatively few of these, that Marlon Brando mired himself in Hollywood and that money was never found for Olivier and Leigh to film the Macbeths. And just as outrageous that in a Kennedy Center season that may be filmed for TV, this "Medea" is the single production to be ignored.

Whatever, we must be thankful for the blessings we get, and we have them in Caldwell and Plummer.

At the Eisenhower Theater, Zoe Caldwell's Medea is a performance of awesome daring and ingenuity. It is daring in and of itself and also that she is doing it with one of the century's most memorable Medeas, Judith Anderson, now playing Medea's Nurse.

The assurance with which Caldwell goes about her bold concept--which imaginatively explains why she would hack her two sons to death--is nothing less than brilliant. This is one of those performances you'll remember a lifetime and is why performers are recalled, years later, through the great historic parts. Comparisons can be made, reflecting the tastes and attitudes of different generations.

The challenge for an actress portraying Medea is to distill understanding for a woman who would kill her children. The Greeks shuddered over her--and over the gods, who since have lost some status. In today's world we read of such horrors in the press and can conclude only that the woman was a lunatic.

Part of the problem in finding nobility in Medea lies in language. Translators for different generations have attempted to suggest the poetic Greek of Euripides. Four hundred years after him, Seneca would have a variation for the Romans and more sympathy for Jason. Anouilh saw Medea as a Russian gypsy who burns her own caravan. Maxwell Anderson saw her as Oparre, an African princess in "The Wingless Victory" who goes off with her sea captain to his native New England, where she murders their children. Katharine Cornell played it. I'll never forget her beauty, but the role was less memorable.

The Caldwell version is by Robinson Jeffers, who wrote it for Dame Judith nearly 40 years ago. His poetry is less florid than the Gilbert Murray script--a standard that served America's Margaret Anglin and England's Sibyl Thorndyke--and heavier than lesser known later versions. It is notable for its searing clarity. Jeffers ignores the final Euripidean vision in which, aloft on the chariot of Zeus, Medea bears the bodies of their children for burial far from Jason.

What has Caldwell the actress to contribute? Aware how audiences make their personal translations within the context of their times, she gives us a blazingly barbaric Medea, someone far beyond the horizon of the Greeks, fully capable of the magic with which Jason could obtain the Golden Fleece. She is not a Greek and, ill at ease in their terrain, expresses natural force against the serene civility of her husband's people. Oddly enough, this is a new accent.

The concept is wholly unlike Anderson's. Her Medea had tried to assimilate a civilization she respected. She was the outsider who had made it to the inside. Caldwell's Medea remains the outsider, battles for her individuality and scorns what to her are the new mores. Africa looking at the rest of the world? The Uzbeks scorning directions from Moscow?

Choosing this image, the actress goes at it with no holds barred. She writhes, undulates, caresses her thighs. Her voice never hits the same tone twice in succession. Blazing from within, her eyes see only a vengeance invisible to others. Her passion is both exotic and believable, and mainly because of her, so, too, is the production.

Time was, actors relished alternating in the roles of Othello and Iago, though now in the United States it would be surprising for Othello to be acted by anyone but a black.

The James Earl Jones Othello, seen earlier at the Warner and now playing at New York's Winter Garden, is seen as our finest since Paul Robeson's World War II performance (to Jose Ferrer's Iago and Uta Hagen's Desdemona). Helen Hayes wrote of that production that it was "one of the finest nights I've ever spent in a theater."

Iago's intellectual vibrance always has been noted, but to that Christopher Plummer adds his actor's reflection that Iago personifies evil. Like Caldwell in "Medea," Plummer begins at a daringly high pitch. Can he maintain it? He does so through outrageous, mocking amorality. And the sensuality of his acting implies another suggestion: that Iago's lust for his superior subtly adds to Othello's discomfort.

What Plummer is doing is using his audience's awareness of contemporary insights. The audience's preconceptions of our sorry historical period are being used by an alert artist to shattering, theatrical effect.

The first Iago I saw--Brian Aherne, playing opposite Walter Huston's Othello--was silkenly deceptive. The most brilliant, cutting Iago I've seen was Frank Finley's, opposite Olivier's Othello, which caused such a commotion in the racially awakening '60s. Olivier chose a melodic Caribbean lilt; Finley's Iago had villainy, but far more guilefully concealed.

Then there's the soldierly approach, in which Iago is portrayed as a bypassed non-com. This is how Ray Reinhardt seemed to see him in Arena's 1963 version with Brock Peters.

Once again, like Caldwell, Plummer has chosen to go for larger than life in his shattering impersonation of evil.

Thus, the great roles do give us the measure of the player. The most admired Lear of recent times--and admired by men such as Olivier and Gielgud, who have played him--has been Donald Wolfit, now memorialized by his dresser-turned-playwright in Broadway's "The Dresser." Noted for keeping shoddy company so that he might the better shine, Wolfit remains vivid two generations later for his anguish. Morris Carnovsky's Lear probably is America's recent finest.

These roles can be likened to proving grounds: An actor can fail as well as shine in them. This season's Macbeth by Nicol Williamson was a disaster, quickly removed. But it might have been expected from an actor who hadn't mastered his Hamlet, either, when President Nixon had the misfortune to present it in the East Room. Theater people believe that there's a curse on "Macbeth," and one can believe it.