Washington — Elizabeth Ashley gives her most riveting performance not on stage - although she is brilliant as the star of ''Agnes of God'' - but backstage, after the red velvet curtain has dropped.
During an interview as headlong as a roller derby, she quotes from Albert Einstein, Sir Francis Bacon, and George Bernard Shaw, as in: ''Shaw said that there's nothing more horrifying than ignorance in action, and I believe it. . . .'' But her best lines are the ones she tosses off herself:
''I'm nomadic. I fire my life every five or seven years.''
''Memory loses its edge. That's why nostalgia is so boring.''
Her voice in the shadowy dressing room is low and husky, as though marinated in honey and smoke. She has just unleashed that compelling voice on a stage blazing with light for nearly two hours as the star of ''Agnes of God.'' (''Agnes'' has gone from a successful run at Kennedy Center to the Morris A. Mechanic Theatre in Baltimore, where it plays until Jan. 28.)
Ashley sits, Chinese-tailor style, on the oatmeal tweed couch in her dressing room at Kennedy Center, and unwinds by improvising her own dialogue.
She has changed from the severe rust wool suit, silk blouse, and pumps of the psychiatrist she plays in the drama into something more Ashleyesque: a black knit top, cut jaggedly on one shoulder, ''Flashdance'' style; long black satin pants; and bare feet.
A cascade of mink brown hair spills in waves around a pale face with high cheekbones and large, inky eyes. From time to time she pushes the waves back with one long, pearl-fingernailed hand, but it is like pushing back the tide.
Interviewing Elizabeth Ashley is like trying to put fog into a box. You can ask questions, but her words roll over them, in a long, witty monologue that suggests James Joyce's stream of consciousness. There is the sneaking suspicion that she doesn't want to give a bad performance, even as herself, and works hard to avoid boring an audience. Even an audience of one.
She confides a few minutes into the interview that ''I see myself as a character. I oftentimes wish that I weren't me, because I mean I'm somebody, (but) all actors, all performers, we make ourselves up. The only thing that separates us from other people who don't know who they are is that we don't accept who the world tells us we are. And we have too much energy to wander around searching for our real self, so we just make up five or 10 people to be.''
She has been Maggie the cat with nine lives, the ''Cat on a Hot Tin Roof'' yowling with love and desperation in Tennessee Williams's steamy Southern drama. And she has been Shaw's Cleopatra, the hussy in ''The Carpetbaggers,'' and the giddy young bride in ''Barefoot in the Park,'' which Neil Simon wrote just for her. She has also had top billing in the lives of two actors, James Franciscus and George Peppard, her first two husbands, before a third and she says last marriage to a musician she identifies only as McCarthy in her book ''Actress: Postcards From the Road.''
About that book, a gutsy and extremely candid best seller, Miss Ashley says she wrote it that way because ''I had read one too many of those it's-lonely-at-the-top, tragic-to-be-a-star, difficult-to-be-so-special, oh-I-went-to-this-fabulous-party-for-this-witty-person-who-said-this-chic-thing books.
Her voice drops half an octave, somewhere between Tallulah Bankhead and Carol Channing's growls. ''And they all miss the point of who we are and what we do. That's why I said, 'OK, let's do it . . .let's go for it as though we were going to make a documentary film.'
In ''Agnes of God'' Ashley plays Dr. Martha Livingstone, a court-assigned psychiatrist sent to a convent to assess the mental state of a young nun accused of murdering her newborn baby.
Miss Ashley's performance as the probing, cynical psychiatrist is a glittering tour de force.
Ashley has performed the role for 18 months since the play won a Tony for Amanda Plummer as Agnes on Broadway. It's now on its national tour with Mercedes McCambridge (Mother Superior), Maryann Plunkett (Agnes), and Ashley in the leading roles.
The star of the play, a self-admitted skeptic, was raised a Baptist but says ''I was never church-religious. . . . I've always gone one on one with God. I figured that God and I had a perfectly good understanding. . . .''
Does she, like Dr. Livingstone, believe in the existence of ''an alternate last reel'' in life? She smiles enigmatically, then returns to the latest reel in her own life:
''I'm sort of in 'park' right now. I ran out of patience. I ran out of love. I ran out of money, and I ran out of time. Mainly I ran out of money. At the age of 40 I wound up with a house in Santa Monica, two Mercedes-Benzes, a Ferrari, and $83. Eighty-three dollars! Thank you very much to those geniuses they call business managers.''
Ashley was born in Ocala, Fla., to parents who were divorced when she was still an infant. Her mother moved them to Baton Rouge, La., where she later went to college free at Louisiana State University, and dropped out after two semesters.
''My major? Escape . . . I knew that I was going to get out (of town).'' Exit lines are her specialty.
There have been lots of alternate reels in the life of this actress who lit out of Baton Rouge for New York and the Neighborhood Playhouse to become a star at 21. The youngest actress to win a Tony, for ''Take Her, She's Mine,'' she ended up on the cover of Life magazine in 1963 as star of ''Barefoot'' just as she was folding up from overwork.
After the failure of her six-year Hollywood marriage to Peppard, the father of her son, Christian, she picked up the shards of her career and started doing television to pay the bills. Her name also became a household word after sizzler interviews on ''The Tonight Show'' over a dozen years. Her films have included ''Ship of Fools,'' ''Rancho Deluxe,'' ''92 in the Shade,'' ''Coma,'' and the recently shot ''Svengali,'' with Peter O'Toole.
But along the way she has dropped out as often as she has faced the camera or the stage, turning her back on the relentless upward climb toward ultimate stardom to chart her own personal course. During the long periods when she has dropped out it has been to sail to a little-known former pirate's island in the Caribbean called St. Bart's. The Creole culture there reminds her of Baton Rouge.
She plans to work two or three more years ''and then my fantasy is to sail down the Pearl River in China, into the China Sea, and then sail down the East coast of Africa . . . that's about five years . . . hang out in the Seychelles, Madagascar, Mozambique, Kenya. . . .''
Meanwhile, it's the discipline of Dr. Livingstone eight times a week, on to Dallas, Houston, San Diego, Los Angeles, and the taping of a cable TV production of ''Cat on a Hot Tin Roof.''
As an actress, she says, she ''seeks to be possessed (by the character) . . . and that's ideal if you can reach that. Because then you get thought out of it altogether. You see, acting is not feeling, believing, thinking, it is behavior. To act is to behave, spiritually, emotionally, physically, whatever.''
On the subject of marriage she has a few parting shots: ''Marriage is a partnership. You didn't get married because . . . you were madly in love, or were looking for a partner in crime. Love is not a good enough reason to get married. . . .
''I think togetherness destroys more relationships than anything else. But we're so processed to believe that presence is a proof of love, and love has become an ego sport. It's all too convoluted. It's not people's fault. It's civilization. And I'm not a fan of civilization.'' Not a bad exit line to get out of town on.