A masterly escorted tour through Shakespeare with Ian McKellen

Ian McKellen Acting Shakespeare The title is direct and descriptive: ''Ian McKellen Acting Shakespeare.'' But it falls short of comprehending the contents of the marvelous show Mr. McKellen is putting on at the Ritz Theatre as he celebrates the Bard. This is a tour de force with footnotes - personal reminiscences, historical references, exchanges with the audience, and those theatrical anecdotes that nobody tells better than an actor.

Mr. McKellen sets the informal tone of the occasion by making his first entrance down the aisle and onto the stage. Dressed casually in dark slacks and an open-collared white shirt, he introduces himself and begins his escorted tour through the worlds of Shakespeare's stage. His only prop is a stout armchair. Skillful lighting changes to suit the changing moods.

In a Playbill preface, Mr. McKellen writes that he hopes to ''convey the thrill of (Shakespeare's) theatricality'' and to share ''the beauty and subtlety of Shakespeare's text.'' His aim is to keep alive the spirit of ''a playwright whose characters and stories never date, whose analysis of human affairs, emotions, and politics is constantly relevant.'' The stated aims are eminently served in an entertainment that combines the features of a superb acting demonstration and a lively theatrical seminar.

Having traversed the seven ages, Mr. McKellen remarks: ''That was Jacques from 'As You Like It.' I'm hoping to be as you like it.'' At the preview I attended, an audience that included many young people left no doubt about the fulfillment of his hope.

In the fashion of the format, the actor intersperses his excerpts with bits of personal history. Before launching into Henry V's famous speech at Agincourt (''Once more into the breach . . .''), he recalls how, as a terrified schoolboy, his delivery of the speech to a ferocious examiner (a British brigadier who hated actors) got him into Cambridge University. He stayed for three years and acted with a number of now famous contemporaries.

Mr. McKellen excels in the actor's gift for visualizing a moment or a scene with a few telling gestures or movements. His English body is a vocabulary of body English. He mimes Burbage arriving in Stratford astride his horse and then suddenly becomes young Will chasing after the company of players - ''meeting the actors and changing his life - and ours - forever.'' In an equally rapid transition, he is an overblown Polonius, a Hamlet turned theatrical director, advising the players, and then soliloquizing in an anguished stream of conscience.

The characterizations multiply as the portraiture fills the stage with changing, vivid images: an asinine north country Bottom, a saturnine Gloucester (later Henry IV), Prince Hal and Sir John in their mock trial scene. One of the most touching passages is Mistress Quickly's description of Falstaff's death. The McKellen Romeo and Juliet are youthfully passionate and somehow contemporary.

Mr. McKellen strikes the contemporary note again in observing that a king without his crown is like a modern star without his name. In presenting the broken, deposed Richard II, the actor recalled the deeply emotional response to the dethronement scene from a Czech audience shortly after the ''Prague spring'' had been crushed by Soviet tanks.

The British star takes a look at two critics (Pepys and Shaw) and creates his own comic critique of David Garrick. Finally, referring the audience to the printed program text, Mr. McKellen explores an actor's ways and means with a fascinating analysis of Macbeth's final soliloquy - its complex of meanings and special significance for the actor, ''a poor player/ That struts and frets his hour on the stage,/ And then is heard no more.''

''Ian McKellen Acting Shakespeare'' concludes with an audience-participation epilogue that punctuates the performance with a delightful exclamation point. It shouldn't be missed.