A MAN LOVE'' is an unaccountably bad film from Diane Kurys, who at other times (with ''Peppermint Soda'' and ''Entre Nous'') has seemed one of the most promising new directors around. The story centers around a very important American film star played by Peter Coyote, which is to say that ''A Man in Love'' is misconceived from the very start.
Mr. Coyote is never convincing as the sort of fellow who draws a large, adoring throng of reporters when he deigns to reveal the plans for his latest project. But at the beginning of ''A Man in Love,'' he announces to the assembled multitudes that he will be making a biographical film about the Italian writer Cesare Pavese. Solemnly, Mr. Coyote describes the details of Pavese's life, right up to the point when he ingests a lethal dose of sleeping powder. ''And he drinks. And he waits. And he dies,'' Mr. Coyote says with glum relish. From this, and from the rest we see of ''Pavese,'' it's pretty clear that the finished product will be keeping them away in droves.
''A Man in Love,'' which is Miss Kurys's first English-language film and which opens today at Cinema 1, is about the romance that springs up between the big star Steve Elliott (Mr. Coyote) and the beautiful young actress who is cast as his co-star. She is Jane Steiner (Greta Scacchi), who is first seen vacationing prettily in Tuscany with her gorgeous mother (Claudia Cardinale) and alcoholic father (John Berry). The father, a hack journalist, later turns up on the set at the Cinecitta studio in Rome after Jane is hired for ''Pavese,'' and he pesters Steve Elliott for an interview. Steve says no, and Jane has the kind of tantrum which, in a film like this, can really pique a man's interest. ''Who do you think I am?'' he asks Jane angrily. ''Who do you think you are?'' she counters. The screenplay, by Miss Kurys and Olivier Schatzky, has a persistently tin ear.
The point, once Steve and Jane get down to business, seems to be that glamorous movie people can't help falling for one another when they're on location, eagerly forsaking the realities of everyday life and letting their lives and art intertwine. This will not be news to anyone who has ever seen a movie set or Francois Truffaut's ''Day for Night,'' or even a soap opera or two. Still, Miss Kurys presents it as revelation, which gives her film a painfully self-evident quality all the way through. The only refreshingly unaffected character is Michael (Peter Riegert), who accurately describes his role as ''Steve's slave.'' Michael takes a cheerfully resigned approach to duties like lounging around the swimming pool, keeping Jane away from Steve's wife, Susan (Jamie Lee Curtis), and remaining at the dinner table alone when Steve and Jane abruptly disappear and go off to bed. When this happens, Michael petulantly calls out for dessert.
Miss Kurys directs the bedroom scenes with more enthusiasm than she brings to the rest of ''A Man in Love,'' and indeed, they are the film's only real means of attracting an audience. Miss Scacchi's languid abandon in these scenes is enough to make up for her more awkward manner elsewhere. But Miss Kurys's way of playing life-versus-art tricks with the audience can be annoying, as when Steve begins to embrace Jane, then suddenly begins howling and curls up into a fetal position; this is then shown to be a scene from the film within the film, though it's directed in an entirely inappropriate way. There are far too many indications throughout ''A Man in Love'' of Miss Kurys's eagerness to find more in this material than is actually there.
In the last reel, the focus suddenly shifts to Jane's mother, whose white blood-cell count has been mentioned ominously throughout, and to Jane's attempt to write down an account of her feelings. Here and only here does ''A Man in Love'' at all resemble Miss Kurys's other, better, less strenuously commercial work. Body Language A MAN IN LOVE, directed by Diane Kurys; original screenplay by Miss Kurys, with co-adaptation by Olivier Schatzky; director of photography, Bernard Zitzermann; edited by Joele Van Effenterre; music by Georges Delerue; art director, Dean Tavoularis; produced by Michel Seydoux, Camera One and Alexandre Films; released by Cinecom Pictures. At Cinema 1, Third Avenue at 60th Street. Running time: 108 minutes. This film is rated R. Steve... Peter Coyote; Jane... Greta Scacchi; Michael... Peter Riegert; Harry... John Berry; Bruno... Vincent Lindon; Pizani... Jean Pigozzi; Sam... Elia Katz; De Vitta... Constantin Alexandrov; Paolo... Michele Melega; Dr. Sandro... Jean-Claude de Goros ; Julia... Claudia Cardinale; Susan... Jamie Lee Curtis.