The deal with actor-director Kenneth Branagh and his wife, actress Emma Thompson, is that they refuse to be photographed together. They will not be interviewed together. And they respectfully require assurance that any separate interviews that do take place will not be cobbled together in a cheap attempt to make the two appear as if they ever were, in fact, in the same room finishing each other's sentences, tapping each other's arms, and laughing lightly. This, however, seems an unnecessary caveat, since Branagh and Thompson-Ken and Em to their friends, the director-star of Much Ado About Nothing and the Academy Award-winning star of Howards End to their industry colleagues-have rarely had a chance lately to be in the same room at the same time, so busy have they been pursuing their separate stellar careers. And yet, even on separate continents, they make a formidable pair -a model of cool togetherness in a business that lends itself to overheated entropy. ''I believe there is a point up to which, of course, anybody would be interested in a situation like (ours),'' says Branagh, 32, defining the limitations he puts on discussion of the union often described, in hyperbolic theatrical terms, as golden, in the tradition of Lunt and Fontanne, Olivier and Leigh, Cronyn and Tandy. He speaks in springy, animated sentences, grounded by his classical training at London's Royal Academy of Dramatic Art. ''But after that point, it's something that not even I believe in, which is us as a sort of joined-up da-da-da. You know, I don't want people buying into some kind of Burton-Taylor double-act thing.'' Branagh is talking in a cream-colored hotel room on an early May Monday in New York, where he is promoting Much Ado About Nothing, his sun-splashed movie adaptation of the Shakespearean romance in which Thompson and he play Beatrice and Benedick, witty, sharp-tongued antagonists obviously made for each other. (Since the chat, the movie has confidently established itself as the high- class hit of the season, with a gross of $5 million and rising.) He has just flown in from Stratford-on-Avon, where he starred in a Royal Shakespeare Company production of Hamlet. This fall he begins work on a new big-studio production of Mary Shelley's Frankenstein, coproduced by Francis Ford Coppola, in which he will play the doctor opposite Robert De Niro as the creature; he will also direct. Branagh is pale with soft flesh, rich pale-honey hair, negligible lips, and the fair skin of his native Belfast (his family moved to the London suburb of Reading when he was 9), and he lounges with grace and stage presence. ''I, as a director, have chosen to employ Emma,'' he explains, ''not because she's my missus, but because she really is a very fine actress and someone who combines great wit and intelligence with a capacity to convey very strong emotion. And you don't often find that combination. She's undeniably one of the best we have. And I enjoy her company, obviously.'' Five days later one commutes to London to find Branagh's missus, 34, in a fancy hotel suite, flat on the bed with a back ailment and talking about a form of domestic bliss. ''This is an island of pretty bad weather, and we're pretty spotty because we tend to like comfort food,'' Emma Thompson says happily. Her hair is slicked back, her wide, clear eyes are professionally smudged and mascara'd, and her vocal inflections go up and down-very arty, very smarty. ''I mean, Ken and I, both of us, when we're in L.A. we eat incredibly healthily and we always lose weight. Then we come back here and immediately become sort of butterballs because we have a terrible habit of sometimes eating sausage in our laps in front of the telly and behaving sort of badly in every way if you want to be lean and gorgeous.'' She does not sound concerned. She sits up to sip a glass of champagne. Because of her back flare-up, she is missing the New York premiere of Much Ado; instead, she celebrates with telephone reports from New York. Having just completed filming James Ivory's adaptation of Kazuo Ishiguro's Remains of the Day, due out in November (once again playing opposite her Howards End costar, Anthony Hopkins), she is off to Ireland to shoot In the Name of the Father, about an IRA bombing, working with Daniel Day-Lewis and Jim Sheridan, the director of My Left Foot. This is all very lovely, but it is not like working with her husband. ''We have a shorthand, which is nice,'' she says. ''Although I probably give Ken more trouble. And he's very patient. He'll say, 'You can't do that!' and I do it and it's 'Oh, all right.' There's no game-playing. It's an easy process, really.'' Ken met Em in 1986 on the set of Fortunes of War, a BBC miniseries aired here on PBS' Masterpiece Theatre. He was a precocious talent who had already made a big noise as the youngest actor ever to play Henry V in the Royal Shakespeare Company. She was a Cambridge-educated comedian from a stylish, theatrical family. (Her late father, Eric Thompson, was an actor and writer; her mother, actress Phyllida Law, plays Ursula in Much Ado.) In 1988, Branagh cast Thompson as his French bride, Katherine, in his masterful film adaptation of Henry V; he married her in 1989 after filming was completed, then cast her as an amnesiac in his 1991 romantic thriller, Dead Again, and as a wallflower who blossoms in last year's Peter's Friends, where once again he acted and directed. Quite Branagh-less, Thompson also won fine notice for her work as a comically lusty nurse in The Tall Guy (1990) and as the Duchess d'Antan in Impromptu (1991), as well as in numerous British television comedy appearances (well, the 1989 BBC sketch series Thompson was her one big flop), and for a nutty star turn as Frasier Crane's first wife, the Raffi-like Nanny Gee, last year on Cheers.