PERHAPS BECAUSE IT happens to be situated in the midst of the city's most infamous slum, The Citizens' Theatre in Glasgow refuses to look at the classics in the usual way -- politely.

Its production of "The Merchant of Venice," for example, was set in Venice circa 1938 with swastikas on the wall. Bassanio was a visiting German soldier, Portia was a tall, blond Aryan, who fell for him, and the anti-Semitism in the piece was, in the words of an onlooker, "absolutely rammed home."

"Hamlet" took place in a madhouse. Hamlet, Horatio and Gertrude were played as if they were in therapy class. The other characters were doctors and interns. Ophela was a nurse who went made.

As for the Citizens' version of Thomas Middleton's "The Changeling," one British critic compared it to "watching the rejected footage of an over-exotic spaghetti Western."

All of which would be of interest to Glasgow or the Glasgow-bound, if Giles Havergal, the 44-year-old, 6-foot-3 Scot who runs the Citizens, were not hanging out these days at the Folger Theatre. For its fourth production of the season, the Folger has not only imported the script of "Marriage a la Mode" from the Citz, as it is sometimes called. It has also engaged its director -- the first foreign director to work at the Folger -- and, one presumes, his decidedly original theatrical imagination.

This may be referred to as branching out. On the other hand, it may just be an instance of tripping down a garden path. One fact is indisputable as "Marriage a la Mode" gears up for tomorrow night's opening: The Folger has embarked on its least conventional undertaking in years.

The play has been fashioned out of two relatively obscure works by John Dryden, a 17th-century English dramatist whose name springs only to the minds of academics these days. About half of the original "Marriage a la Mode" is a comedy, in which characters with more money than sense gibber on about marital infidelities. The other half is high-flown tragedy, heavy on the histrionics.But never, Havergal discovered, do the two plots meet.

His first move was to replace the tragic half with "All for Love," Dryden's better-known Anthony and Cleopatra drama. Then he whittled both comedy and tragedy down to size. Finally, he hit upon a solution that made the seemingly unrelated parts fit together. He turned "Marriage a la Mode" into a backstage comedy about a troupe of petty actors rehearsing an impossibly eloquent tragedy. The result was an unexpected hit, when it was first performed at the Citizens 18 months ago.

"Of course, I feel odd doing the play here," admits Havergal, whose speech tends to suggest soft machine gun fire. "But no odder, really, than I did when I staged it at home. It's a genuine experiment every time. Dryden's work is not widely done, and when you get off the beaten track of the usual three or four Restoration comedies, you're in strange country. A whole evening of Restoration comedy is as irksome as a whole evening of high classical tragedy. But the blend, that's what's fascinating. Now you get that marvelous contrast of idiotic actors larking about and then suddenly going onstage and being extremely grand. It seems to me that the theatre, better than any other medium, can be fun and serious at the same time. Shakespeare manages so wonderfully. If you're going to have a live entertainment for live audiences, it must contain things for the mind, the heard and the funny bone."

The kind of heightened contrasts that Havergal is pursuing in "Marriage a la Mode" is the very stuff of the Citizens itself, which he and two codirectors have piloted since 1969. Largely unknown in the United States, but widely hailed in Europe, it seems to combine distinct aspects of the Tyrone Guthrie Theatre and Charles Ludlam's Theatre of the Ridiculous. Its fare is highbrow, mostly classics and contemporary masters. But its approach is resolutely populist, when it is not iconoclastic. "No British company matches their visionary skills in the reclamation of a classic repertory," the London Times noted recently.

Says Havergal, "In a city with a lot of social problems, we have to fight for the right to do the plays of Goldoni, Shakespeare and Webster, as opposed to those set in a tenement. The point is the classics, thought to be so stylish, really were written for a popular audience. We try to appeal to the yob [ruffian] and the snob and also to the yob and the snob in each one of us."

The Citizens certainly had no qualms about turning Marcel Proust's "A la Recherche du Temps Perdu" into a 3 1/2 hour stage play. ("And a rather popular one" adds Havergal.) Then there was "The Threepenny Opera." "It began," says Havergal quietly, "in a beautiful pale blue and gold turn-of-the-century drawing room, with draped piano, which an elegant elderly matron sat playing. Her butler and maid served her with a light supper on a tray. Suddenly, the place was invaded by a gang of toughs, led by the maid and butler, who wrecked the joint, spat on the matro, p----d on her and ultimately killed her, then performed the opera in the wreckage of the room."

The Citizens' production of "De Sade Show," based on the writings of the wicked marquis, opened with a man in a pink jockstrap, diamond earrings and a dog collar striding onstage. The ensuing drama pitted the good girl against the bad girl in the arenas of religion, law, politics and finance. In each case, the bad girl emerged victorious.

It is also rule of thumb at the Citizens that anyone in the youthful company can play just about any part, if he's good enough. Cleopatra and Lady MacBeth have been acted by men, while the male chauffeur Matti (in Brecht's "Puntila and Matti") was a woman. "We've played so much in drag over the years," says Havergal, "that the audience simply accepts it now. We just pick the best performer for the part, irrespective of age, sex, whatever. It's not a sexual statement we're making. What we're doing is emphasizing the artifice of the theatrical experience. We don't pretend to naturalism. What we're really saying is it's not an old granny up there on the stage. It's an actress -- or indeed, an actor -- playing an old granny. The charge the audience gets is in the process of imitation."

The freewheeling approach to the sacrosanct, combined with ridiculously low prices (general admission is $3; students can get in for $1.50 and the unemployed for free), has made the heavily subsidized theatre a smashing success. "The form is often so striking," says Havergal, "that some people can't or don't want to believe that there is anything behind it. Those who don't like us usually charge us with gimmickry or camp. But what is camp? Form without content. Almost everything we do is rooted in some kind of conceptual idea. Nothing is done just for the sake of doing it. People are more attuned to ornate verbal and visual images than we give them credit for. They will allow you to do something extraordinary, if you can fill it 100 percent."

Even the Citizens' quarters, a 104-year-old Victorian playhouse, is excessive with contrast -- drab on the outside, a riot of gold, black and scarlet on the inside. "It makes the statement: This is a theater. It couldn't be anything else," he says. "Unless it was a brothel."

Loquacious as he is on the theatre, Havergal all but dries up on the subject of himself. "What is interesting, if anything is," he says, suddenly flooded with reserve, "is what I do, not what I am. When my work is over, I tend to be quite isolated. Reading is a prime sport. I wish I could say I ride a bicycle or bake meringues. I don't."

Still, he admits, under prodding, that he was born in Edinburgh, which "is helpful" because it protects him in Glasgow from the charge of being "some degenerate southerner, ruining our theater by bringing in all this foreign rubbish." He went to Oxford, plunged into the theater as an assistant stage manager shortly after graduation, and hasn't been out of work since.

For the Folger's artistic director, John Neville-Andrews, Havergal's presence presents the first sally in the battle to "stretch the company and our audiences." If the experience proves fruitful, he says, he's got half a dozen other foreign directors he'd like to tap. "It's important to expose this theater to something different, not just stand still," he says. So a policy may be at stake here, not just a curious play.

Havergal puts it differently. "Our work at the Citizens has a strong intellectual backup, which is important for a place like the Folger. The challenge is to use that academic basis as a springboard for a very flamboyant presentation which will catch the audience, I hope, at a very odd angle, indeed."