An actor's life is rugged, but one of the small consolations is that it is above economic cycles--unemployment is common in good times or bad. Perhaps the closest anyone has ever come to having a regular acting job in the theater is Marian Seldes who played the part of Myra Bruhl in everyone of "Deathtrap's" 1,739 performances on Broadway. That is a record of job security noted in the "Guinness Book of World Records."

"Deathtrap" closed last Sunday, and several days later a curious telephone caller found Seldes not particularly bothered in her newly unemployed state. "It hasn't sunk in at all," she said. "This is the first evening in 4 1/2 years that I have had two nights in a row at home." Where is she signing for unemployment, she is asked of the social ritual that is as much a part of an actor's life as an opening night party or a false smile at an audition? "Well, I don't have to. You see, I teach at Juilliard and am self-employed as a writer."

During the four-year run of the play, Seldes managed to turn out two books, a work of nonfiction and a novel. Because she was killed every night at the end of the first act, she had plenty of time to wait until her curtain call. So she would go back to her dressing room "and act the part of a secretary," typing what she had written during the day, writing letters or an occasional book review.

She credits the five actors who played Sidney Bruhl, her murderous playwright husband, during the course of the run with keeping her from being bored in her work.

Finally, it is the little things in life that she no longer has to worry about. The current spate of wet weather would have frazzled Seldes to no end just a week ago, but now, "It doesn't matter if my hair gets wet." She explains: "If you're in a play and you're a girl, your hair has to look just right. This is the first time in 4 1/2 years I could take a walk in the rain. I would never dream of doing it before."