The relentless march of nearly 20 years time has left no mark on Christopher Fry's The Lady's Not For Burning. It is the same fat chestnut that it was the day he finished it.

What makes this play a chestnut is not so much its romance, its medieval setting, its aria-like speeches--as its flagrant predictability. The title tells you that the lady won't get burned, just as the juvenile's first glance at the ingenue tells you that they'll end up eloping. Despite the twist with which the story starts out--the hero arrives and announces he wants to be hanged--the gist of the play is not only old, but old-fashioned. Life, it says, is lousy and there are two ways out: Death and falling in love. In the age of No Exit, Fry's play has a chestnutty premise.

But there is another thing about every chestnut: for one reason or another it is dearly loved and long performed. The reason for The Lady's long and happy life is its language. Christpoher Fry has a Chinanman's fascination for high-meaning word plays, mixed with an Irishman's compulsive wit. He cannot bear to write a line, for even the lowliest of characters, which is not pure honey. The flow of mellifluous banter carries the play along, and on it floats truth after home truth. Few writers and fewer playwrights can mix colloquial expressions with genuine poetry as smoothly as can Christophesr Fry. It is natural that he should use an unabashedly romantic, medieval background to make his verse look at home.

Given a chestnut, however, there are two things you can do with it. You can make coupe aux marrons or turkey stuffing. This production, though it has its bright moments, tends to be stuffed turkey.

Director George Hamlin has made the staging posed and mannered, draping his actors in window arches and at ladies' feet, draping their faces with wigs and putty. The actors, for their part, cannot always keep the play's eloquence under control. Paul Glaser, as the hero crying to be hanged, is all forensic and fingerpointing, but often his gestures distract from his lines, and sometimes he loses the thread of the poetry in his forced jauntiness. Nancy McDoniel, the lady accused of witchcraft, smiles and enthuses as constantly as a Dickens heroine with never a trace of the wryness and mystery in her part.

The two principals hardly do the lines justice, but many of the secondaries do. Lawrence Senelick has studied his Pistols and Shallows until he has assembled the whole bag of Shakespearean character tricks, and he executes them perfectly. John Lithgow makes an engaging brother to Tom Jones, who carries off the villain's part with great authority. And Sheila Hart, if she would sharpen her diction a bit, would make a perfect world-weary mother.

The cast is not yet an ensemble and many of the actors are slow on the pick-up. At times the pace is agonizing, for these are lines that must leap at one another like sparks in dry air.

John Braden has designed an effective set, and his lighting is most dramatic. Lew Smith's costumes are, as usual, colorful enough to make a peacock blush and wonderful to behold.