Take the Case of the Third Murderer.

It happens in "Macbeth." The tyrant has hired two murderers to kill Banquo, and at the last minute they are joined by a shadowy third man.

"But who did bid thee join with us?" they ask, as the scene opens in midconversation.

"Macbeth," the man replies.

Some scholars believe this is Macbeth himself, come to make sure the job is done right. Others think that's unlikely because Macbeth later talks to the two killers and seems unaware of what happened. According to this theory, the Third Murderer was sent by Macbeth in his paranoia to check on the first two.

"I've actually seen it done that way," said Sebastian Shaw, a 74-year-old veteran of the Royal Shakespeare Company. "It's an interesting idea."

Shaw and four other British actors are here through March 8 to teach and talk and maybe run through a scene or two for a remarkable program sponsored by the Folger Institute of Renaissance and 18th Century Studies. They will be taking up just such riddles as the classic of the Third Murderer.

As actors in residence -- an idea now in its seventh year -- they are leading workshops and seminars for teachers at all levels, and they're lecturing at the Kennedy Center and the Library of Congress.

One particular area they will explore is that hazy frontier between scholarship and acting.

"Every role can be played in 101 different ways," remarked Shaw, whose acting credits run to three solid columns in "Who's Who in the Theater." "And styles change constantly. If you knew how Shakespeare's actors played the parts, you'd be a great actor."

He laughed to recall his own version of Romeo, vintage 1936, in which he more or less sang the Balcony Scene. The fashion then was to view Romeo as a lyric poet (One remembers Leslie Howard's screen Romeo of that year.). The modern Romeo is more boyish, less consciously articulate.

Aside from the programs for high-school teachers and elementary-school children, the seminars will reach graduate students, university teachers and scholars, and there will be these public events:

"Ariel: Shakespeare's Sweet Power and Music" is tonight at 8. "Shakespeare and the Actors" is Tuesday at the same time. "Murder Most Foul" is Thursday, and "Ariel" will be repeated March 8, all at the library of Congress' Coolidge Auditorium.

Homer Swander of the University of California had the scheme for this rapprochement of the academy and the theater. It is peculiarly apt for Shakespeare, who for most Americans is notoriously opaque when read but who comes gloriously alive when acted well.

And the British seem to have a corner on making Shakespeare sing. People are always asking why this is.

"You can be taught certain skills," said Shaw, who studied at the Royal Academy; "movement, dancing, sword fighting, how to handle your body, control. aBut nobody can teach you to act. It's a gift."

Of course we all act, in a way, he pointed out. But managing it on cue is another thing. The Briton's respect for the language may have something to do with it, too.

Shaw, a doctor's son from Norfolk, started out at age 5 at the Court Theater. He knows all about child actors.

"My daughter was in a school play," he said, getting up to act it out. He showed her glaring over the footlights, transfixed, while the teacher whispered hoarsely from the wings. "Finally she said in a very firm voice, 'I don't fink I'll do it.' And stomped offstage."

Shaw retired at 7, tried his hand as a painter, returned to the stage and the Royal Academy as a teen-ager.

"There was a poor fat boy just enrolled, and we thought, too bad, it's a shame. But in about two days we stopped feeling sorry for him. It was Charles Laughton."

Shaw went on to play everything from Shakespeare and George Bernard Shaw to a number called "A Lass and a Lackey." He is still at it, and he's written a play himself, "The Cliff Walk."

"Actors take themselves onstage, and they absorb themselves into the character, and the character into themselves, and they give you that. It doesn't always work. It's rubbish to say any actor can play any part. At the very least, a fat man can't play a thin one."

He is looking forward to the jam session on "Hamlet," shared by the actors and the academics.

"We'll discuss what it's about and what the choices are and how the people interrelate. And then we'll do a scene, maybe the Nunnery Scene, when all the people are there, Hamlet, Ophelia, Polonius and everybody.

"And we'll discuss it some more, and argue, and read the lines one more time . . . and then finally we'll throw our books away and let go and just do the bloody thing."