Imagine yourself at a party where somebody spots Ian McKellen and asks him to put on an impromptu show for the guests. You make a circle around him, and he begins with anecdotes about his own experience acting Shakespeare, stories about other actors' tribulations with the Bard, tales about Shakespeare the man.
Well, pull up a chair in front of your TV set, get the popcorn ready, take off your shoes -- ''Ian McKellen Acting Shakespeare'' (Monday, PBS, 9-10:30 p.m.) is just that, a superb, seemingly off-the-cuff evening of Shakespearean entertainment. It is a party, a ball. Or, even more like it, a Bardological feast.
A veteran of the Royal Shakespeare Company and the National Theater, Mr. McKellen has played Hamlet, Macbeth, Richard II, and Romeo. During this time he has managed to save up a store of Shakespeare stories and has emerged with some highly original interpretations of the various roles. He'll have you enthralled when he does not have you in stitches.
In ''Acting Shakespeare,'' directed with invisible skill by Kirk Browning, produced by David Susskind, McKellen has succeeded in giving us a kind of Shakespeare vaudeville show, filled with bits of revelation, moments of astonishment at his skill, 90 minutes of pure Shakespearean fun and games. Even if Shakespeare is not your cuppa, you may find that ''Acting Shakespeare'' is fine popcorn entertainment. It'll make you forget you're seeing one of the world's finest Shakespearean actors bringing Shakespeare to life. What you will be seeing is merely a superb entertainment, a marvelously literate variety show.
I chatted with Mr. McKellen the other day when he arrived from his home in London to help promote the show. Far from the formal attire one might expect of a distinguished Shakespearean actor, he was wearing a multicolored striped rugby shirt, faded jeans, and sandals.
''I'm sure Shakespeare would have approved of my little show,'' he says. ''His plays were written to be played with the audience clustered about. Sometimes his characters spoke directly to the audience. That was his way of truly engaging them.''
McKellen is pleased that the informality of his production may attract audiences that might never watch a standard Shakespeare production. ''The majority of televised Shakespeare I see isn't very good. It is under-rehearsed, with far too much emphasis on the visual. I have been disappointed by the BBC 'Shakespeare Plays' on PBS, which are too bound by mistaken tradition. Those traditions are 19th-century theater traditions, not Elizabethan traditions. Shakespeare can be done well on TV, because viewers are used to sitting in front of a screen and listening to Mr. Cronkite or Mr. Rather, aren't they?
''The BBC Shakespeare productions are improving, though, since Jonathan Miller took over. We had been getting wonderful long shots of groupings when the camera should have been up close, revealing what is happening behind the actor's eyes.''
''No. Not at all. I love watching on TV an expert who is in love with his specialty. That's enough. I want to share the enthusiasm and I hope audiences will sense my enthusiasm, too. Bronowski (''Ascent of Man'') had it and so does Alistair Cooke and Sir Kenneth Clarke. All that is needed for my show is a willingness to put 90 minutes aside and see what this chap has to offer.''
''What do I do next?'' he then asks himself. ''I go home, read scripts, and hope to find something I want to do next, something worthwhile, perhaps a film.''