'A Week's Vacation'

French filmmaker Bertrand Tavernier has gained a considerable reputation with such dramas as ''The Clockmaker'' and ''Let Joy Reign Supreme.'' His latest, ''A Week's Vacation,'' is just as leisurely, and just as concerned with developing character rather than plot. But it's a more engaging film, partly because of its appealing background - the city of Lyons - and partly because of Nathalie Baye's attractive portrayal of the leading role.

She plays a young schoolteacher who suddenly can't face her work anymore. Needing to sort out her life and priorities, she takes a week off, spending some of her time alone, and meeting with various friends and acquaintances. These confrontations have little deep meaning in themselves. But through them, little by little, she finds her equilibrium again.

When filmmaker Tavernier and star Baye visited New York recently, it was an opportunity to ask whether they had carefully planned the film, with its relaxed and spontaneous feeling, or whether it evolved during the shooting and editing. The answer was: some of both. While it was being made, Tavernier wasn't sure how the story would end. So he altered the script as filming progressed, relying on his own instincts and those of his performers. Miss Baye, a versatile actress, was happy to join in the freewheeling spirit of the occasion.

An enthusiastic fan of jazz, Tavernier seems to fancy himself a kind of cinematic musician - putting great value on structure, but always willing to improvise, and treasuring the moments when unexpected magic suddenly materializes out of thin air. He puts much faith in impulse and instinct, and trusts that something worthwhile will happen if you just let yourself go with your material.

It's not surprising to learn that Tavernier admires both Charlie Parker and Robert Altman, two artists who have understood the secret of spinning spontaneous dreams on solidly constructed underpinnings. While his new movie never quite reaches the levels of inspiration it yearns for, its intermittent charms are a good sign that Tavernier is headed in the right directions. As for Miss Baye, she is happily fulfilling the promise she showed in such earlier pictures as Truffaut's ''Day for Night'' and ''The Green Room.''