The debonair new resident of 221B Baker Street

THERE is something about Jeremy Brett that makes you want to put the word ``darling'' at the end of every sentence, darling. I mean really, all those Etonian vowels, the West End dressing room with that darling basket of fruit, the darling bouquet of lilies from all those darlings on opening night. ``Oh, I mean really. It's the most divine theater in London, and the show is just a great joy,'' says Mr. Brett with a wave to his dresser. ``See you anon, my dear.'' ``Right,'' says the actor, swiveling to face the interviewer in question, ``Now, tell me....''

Brett is the latest - some say greatest - interpreter of Sherlock Holmes. Ever since Sir Arthur Conan Doyle unleashed the fictional detective on the British reading public more than a century ago, the enigmatic Baker Street sleuth has been portrayed in hundreds of films, radio dramas, and stage plays - not to mention a ballet, a musical, and an oratorio - by more than 100 actors, most notably, Basil Rathbone in those early 1940s films.

Then in 1982, along came Brett with a campy portrait of the cerebral detective in Britain's Granada-TV series, ``The Return of Sherlock Holmes'' (seen locally on the Public Broadcasting System's ``Mystery!'') that all but broke the mold. The 25-story series - one of the few faithful adaptations of Conan Doyle's canon - was a critical and commercial success that sold to more than 50 countries, including the Soviet Union and China. (The six latest episodes, beginning with ``The Sign of Four,'' premi`ere Thursday night on PBS.)

In a very crowded field, Brett was suddenly the definitive Sherlock Holmes. For the debonair Englishman, an often out-of-work classical actor who had migrated to Hollywood during the 1970s, surfacing in lesser mini-series and the road tour of ``Dracula,'' it was the definitive dream come true.

``Oh, it's a dream. Don't be fooled when actor says he's afraid of being typecast. That's rubbish. What are you in the business for? If you're not successful, it's disaster.''

Six years and some 26 TV hours later, Brett is attempting to turn his television popularity into something of a return to the theater. ``The Secret of Sherlock Holmes,'' a new play exploring the friendship between Holmes and Dr. Watson, opened in London last month, conceived by and starring Brett, along with his TV sidekick, Edward Hardwicke, as the unflappable Watson. The drama has received decidedly mixed reviews, but Broadway remains a more than hush-hush option. ``No, no darling,'' says Brett, ``we have to wait and see if we're making money. Otherwise we'll just come off and go to Peru or wherever.''

It is a superlative backstage performance that Brett is giving in his subterranean dressing room at London's Wyndham's Theatre, just a few hours before curtain. The wallpaper is English awful, the air heavy with the perfume of the lilies and the gently decaying fruit. And the contrast between the on-camera Holmes - a reptilian dandy with lacquered hair and pursed lips - and the offstage Brett could not be more, well, theatrical.

For one thing, Brett is enormous. He stands more than 6 feet 2 and if not exactly burly, he is at least bulkier than his whippet-esque Holmes. Even his hands, which the actor uses with rapierlike precision in the films, seem soft, meaty. Brett has dressed for the occasion in a sort of dash-it-all dishabille, unbuttoned double-breasted blue blazer, silk paisley scarf, baggy white trousers rolled at the ankles, and a blue T-shirt that doesn't quite cover his surprisingly ample girth. His hair is an errant shock adrift over his forehead. The only vestige of the world's supersleuth? Brett's throaty, cultured voice (he did in fact attend Eton) that charges up and down the sentences in pianissimo runs.

``I think it was in a taxi that someone suggested my name for the series,'' says Brett, settling into the one of the long Holmesian narratives that will constitute the interview. ``I was going to dinner on Charlotte Street and I was taking my son with me. I got into the car, it was pouring with rain, one of those wonderful February nights ... and I said, `I really don't want to do this. It's been done by so many people.' I was frightened, frightened of failing. I mean I haven't got the brain, and my look was all wrong. It would have meant the most extraordinary makeup. So I said yes.''

The series was an on-again, off-again project for several months, during which Brett did a ``Tempest'' in Toronto and began rereading Conan Doyle's stories. ``I hadn't read them since school and I began to find all sorts of things, like he only wore the deerstalker in the country, a top hat in the West End, and the bowler in the East End. ... Suddenly there was an essence.'' Brett also discovered in the original Sidney Paget illustrations ``a look that I suddenly thought that I could use, particularly in profile.'' (Staying in profile, it turns out, and the whippy use of his hands close to his face ``to indicate a change of thought,'' are two of Brett's main techniques.)

``Then I did a camera test and I was terrible. I was so nervous. To try and give the illusion of intelligence, I painted my forehead white and I had a black, gentian violet under here,'' he says, putting hands to his throat. ``And I think I walked rather fast. My brilliant producer finally said, `Is there going to be anything of Jeremy in this at all?'''

That first episode was ``The Solitary Cyclist,'' about which Brett says, ``The momentum had begun and I had begun to find things - the cracks in the marble - such as his delicacy with women, his failures, the little human elements. ... But he's a very isolated, private man, he's removed emotion from his life, and that's what makes him so hard to play.''

But play him Brett does, in a supremely, if slightly out-of-fashion, high style. ``Of course, the performance is over-the-top,'' says Brett with a laugh. ``But I'll tell you what it is, I've played him as a kind of romantic hero ... because I don't feel adequate for the part. You know, you have all these marvelous ideas in your head and all that comes out is this awful `you.'''

Avoiding being bored with ``you'' seems to be a driving force for this actor, the son of World War I hero and an Irish Quaker mother, who originally wanted to be a singer. But his ``glorious soprano didn't break so well.'' Then came aspirations to be a jockey or a dancer. ``But I got too big to be a jockey. As for dance, after I went to my first ballet I suddenly realized that the man spends all his time lifting the woman in front of himself, and I thought that's a bit deadly.'' Brett settled on classical acting, attended London's Central School of Speech and Drama after Eton, and later worked in the West End.

``It was lean pickings,'' says Brett about those years. ``I was the classic young romantic juvenile, the young gent, just as the Angry Young Men [playwrights] were coming in. I mean, right. No space for me.''

Brett eventually took refuge with Sir Lawrence Olivier at his National Theatre, spending four years churning out credible Shakespearean performances before heading for Los Angeles. There he appeared in such films as ``War and Peace'' and ``My Fair Lady,'' in which he played Freddy and sang ``On the Street Where You Live.'' To this day, Brett says he prefers film to the stage.

``It's this kind of love affair between yourself and this giant one-eyed moose,'' he says about the camera. ``You cannot lie to that all-seeing eye.''

Brett's success as Sherlock Holmes has also engendered a different kind of all-seeing eye. After the death in 1983 of his second wife, Joan Wilson, the Boston-based executive producer of ``Masterpiece Theatre'' and ``Mystery!,'' Brett suffered a well-publicized nervous breakdown - a time about which he hisses, ``The press were vile. The bloody press exposed it, so I had to wear it outwardly and it's been horrid. Now, I am just beginning to get my health back.''

``It's one of things I think you'll see in these new episodes,'' he says. ``He's warmer, a little less on the dark side. I've changed the makeup a bit; he doesn't look so ill, so coked up.'' About Holmes's controversial cocaine habit, Brett says, ``That's the other thing that thrills me about these new stories, that he gets to kick the cocaine habit, which had begun to worry me. Anything that says, `Hello cocaine,' these days, well, a statement has got to be made.''

And then Brett, that darling, darling man leaps to his feet. His dresser has reappeared and the show must go on. But not before he plants a swift kiss on the cheek of his interviewer. One half expects to hear Sherlock Holmes's immortal words, ``The press, Watson, is a most valuable institution, if you only know how to use it.''