A courtroom is not where this Portia is most successful. Kelly McGillis, Brian Bedford star in fast-paced `Merchant'

The quality of mercy is slightly strained by casting movie star Kelly McGillis as Portia in Shakespeare's ``The Merchant of Venice,'' as director Michael Langham has in his innovative production now at the Folger Theatre. Ms. McGillis is the tall, blond beauty who starred in the hit films ``Witness'' as an Amish widow and ``Top Gun'' as a United States Air Force astrophysicist. Between films, she starred at Kennedy Center in the American National Theater production of ``A Sea Gull'' (as director Peter Sellars retitled the Chekhov play). Neither the bird nor the plane nor the thriller roles prepared her for the superwoman challenge of Portia in one of Shakespeare's most demanding parts for an actress. McGillis is an accomplished actress with a stunning radiance on stage, but she is not yet a complete Portia, and until she is, this ``Merchant of Venice'' is often more of a romp than a poetic play.

For Portia must be not only lovely and witty. She must be judicious, authoritative, and imposing enough to convince not only the court but the moneylender Shylock (Brian Bedford) that he cannot have the pound of flesh Antonio, the merchant of Venice, has forfeit for the 3,000-ducat debt he owes. She must have that grave demeanor, that sense of high seriousness known in Latin as gravitas. And she, disguised in men's clothing, must also convince her husband, Bassanio (the friend for whom Antonio has borrowed the money), that she is a young but learned judge.

McGillis as Portia is charming, girlish, giddy (she giggles a lot), and simply smashing looking in her Venetian silks. She makes a lovely Portia in the scenes in which her suitors, according to her father's will, must choose among the caskets of lead, silver, and gold to claim her in marriage. She really cavorts through the role, displaying a special gift for comedy.

But in the important courtroom scene and particularly in that crucial speech to Shylock (``The quality of mercy is not strain'd;/ It droppeth as the gentle rain from heaven/ Upon the place beneath...''), she is not compelling. The mercy speech is the acid test of a Portia; McGillis gets a passing grade, but no A. It may have been opening-night jitters, but her voice seemed uneven, sometimes sounding classically Shakespearean, sometimes light and contemporary. (Last year another statuesque film actress, Sigourney Weaver, played Portia Off Broadway. In contrast to McGillis's girlish Portia, Miss Weaver brought a formidable grace to the role, a womanliness that seemed more in character.)

As a result, Mr. Bedford's Shylock is the true star of this ``Merchant of Venice.'' The British actor gives a masterly performance, slicing into the soul of the Jewish moneylender who has suffered bitterly from the anti-Semitism of Renaissance Venice. Bedford's Shylock, unkempt and shambling when we first see him, turns with loathing on the Christian merchant Antonio and reminds him that he'd called Shylock a dog, a misbeliever, and spat on him. ``And for these courtesies I'll lend you thus much moneys?'' Shylock asks.

Bedford in his probing performance shows us how Shylock's wounds and pride have steeped, along with his miserliness and his grief at the loss of his daughter to a Christian, to create the lethal revenge he wants on Antonio. In one of Mr. Langham's most effective scenes, Shylock delivers his famous ``I am a Jew'' speech as an angry outburst as children taunt him in the street: ``If you prick us, do we not bleed? if you tickle us, do we not laugh? if you poison us, do we not die? and if you wrong us, shall we not revenge?'' Because of this carefully honed characterization, we understand his Shylock in court, as impressive-looking and implacable as an Old Testament prophet in demanding justice.

The spirited cast includes Geoffrey Lower as a handsome, ardent Bassanio, Jack Ryland as a staunch Antonio, and Kevin Black as an impetuous Lorenzo who makes off with Shylock's daughter Jessica, played with 'elan by Michele Farr. Philip Goodwin, who plays Portia's suitor, the Prince of Arragon, as a pompous fop, is delightful in this strong cast, which includes Lucy Brightman as his mother, Robert Jason as a dashing Prince of Morocco, John Seidman as Shylock's servant Gobbo, and Emery Battis as Gobbo the elder.

Under Langham's imaginative direction, this ``Merchant of Venice'' is a fast-paced, entertaining, and vivid evening in the theater. No dozing among the groundlings for this show. Contributing to this effect are the stunning costumes by Susan Hirschfeld and clever set design by Douglas Stein, who manages to turn the Folger's small stage into a marble and wrought-iron impression of Venice. There is also some caroling of Renaissance music by Stephen Douglas Burton in case the music of Shakespeare's words - ``the sound of shadow,'' ``the gusts of heaven'' - is not enough for you.