It's been a long time between Broadway appearances for Stacy Keach. The first one was his starring role in "Indians" 10 years ago. Now, finally, he's back, as the murderous author in the long-running "Deathtrap."

The long absence seems like a commentary on the state of the theater: An actor acclaimed for his Hamlet, his Falstaff and other weighty roles must wait a decade after his Broadway debut to return -- in a commercial thriller. But it's more of a commentary on an actor's need to combat the kind of thinking endemic to the theater that pigeonholes performers in specific cagegories.

"I was thrilled by the opprtunity," Keach said of his "Deathtrap" role, "because I've been identified mostly as a classical, serious actor. I'm pround of it on the one hand, but on the other hand, from a pragmatic point of view and a commercial point of view, it doesn't do me much good."

That "serious" identification has faded somewhat during the past six years as Keach has nurtured a film career in California. In addition to acting in movies, he has been involved with writing, directing and setting up a movie-production company. "It takes time," he observed, "and during that time I never really had much of a chance to do any theater. Every year it would prey on my soul more and more that I hadn't done any theater, until I finally came back with 'Cyrano.' I think that's what finally revitalized my juices and got the ball rolling again."

Keach starred in Edmond Rostand's "Cyrano de Bergerac" for a month last fall at the Long Beach Theatre Festival in California. He's eager to star in a New York production when his chilling activities in "Deathtrap" come to an end. Cyrano has become his favorite stage role, even ranking above Hamlet, which he played in three different productions, most notably one for Joseph Papp's New York Shake-speare Festival in Central Park. "When you're playing Hamlet," he said, "It's such a total experience, it plays you.Itz's hard to leave the character in the dressing room and walk out of the theater." Cyrano, though, is heroic, spirited, idealistic, funny and romantic -- all qualities the 37-year-old Keach enjoys capturing.

Meanwhile, he's having fun as Sidney Bruhl, having replaced John Wood in Ira Levin's comedy-thriller. "People have asked me, "Don't you feel a little strange taking over for somebody?'" Keach said. "But I don't, I really don't, because I don't think there's a stigma attached to it for a couple of reasons: One, some pretty prominent people have done that -- Richard Burton [in 'Equus'] in New York. And two, I think, having an identity as a classical actor... one of the things you do in your profession is you play parts that other great actors have played. I mean, I've played Hamlet and Peer Gynt and Falstaff and Coriolanus, Cyrano de Bergerac, parts that a lot of other actors have played, too."

He's also been asked by people if he wasn't intimidated by the idea of taking over for an actor of Wood's virtuosity. "And I say, 'Well, I would have been if I'd come to see John's performance the first day I arrived in New York. I wouldn't have dared to set foot on the state.'" But he saw his predecessor only after having rehearsed and gotten his own conception of the part -- unlike the British Wood, he's playing Sidney Bruhl as an American, with a different set of mannerisms.

As he sat in his East Side hotel suited discussing acting -- and such wide-ranging interests as roller skating, horseback riding, science, his study of spiritual philosophy, playing his electric piano and writing from midnight to dawn -- Keach seemed like the embodiment of the soud-in-mind-in-a-sound-body concept. He has clearly given much thought to the kind of roles he's played in the past that have stamped him as a serious actor -- and his efforts to remove the stamp.

"I've never thought of myself as being a serious actor in that sense," he observed. "I started off in 'Macbird,' which was a political satire. Comedy is really the thing that I do best. I like it, and happily, I've been doing a lot more comedy." But he added, "I've had very little opportunity to do comedy in movies except for heavy kind of caricature films. I just did a thing with Cheech and Chong called 'Up in Smoke'; I play a narcotics officer.... But beyond that kind of overt caricature, I've never had a chance to do the kind of Noel Coward comedy, in terms of a David Niven or Cary Grant -- that type of light, comedic kind of thing."

In his next movie, William Peter Blatty's "The Ninth Configuration," Keach has still another serious role -- as a man who's believed to be a psychiatrist checking up on emotionally disturbed servicement. "For me it represents the end of an era; I'm really trying very hard not to do the intensely serious or heavy kind of parts," he said.

Most of his movies have called for just that, among them the intellectual "End of the Road" and John Huston's "Fat City;" in which he had his favorite screen role, as a second-rate prizefighter. The latter, although a critical success, was a commercial failure that made it difficult to get other roles for awhile. By contrast, "Up in Smoke," which he calls a silly movie, has made more than $50 million. "And there's no comparison between 'Fat City' and 'Up in Smoke,' but I'll get work as a result of 'Up in Smoke.'"

Partly to shake his serious-actor image, Keach starred during the 1974-75 season in the ABC-TV adventure series "Caribe." But the movie backfired. "I've come to realize that when you do something well and you're recognized for it, to work against it doesn't really help you," Keach said. "I never thought of it in those terms at the time; I thought of it as an extension, moving off in another area, trying something else. But a lot of people interpreted it as being a sell-out. I was shocked. But when that series was over -- it was a nine-month period -- the phone didn't ring. I mean I couldn't get arrested."

Now he's been branching out into other areas. He has directed for public television, beginning with a documentary on prisons which he wrote, called "The Repeater," and has written an as-yet-unproduced screenplay. With his brother, James (his costar in a TV film about Orville and Wilbur Wright)8 he has a production company whose first project is to be a movie called "The Bandit Kings." Plans call for Stacy and James to play Jesse and Frank James; David, Keith and Robert Carradine to play the Younger brothers, and Robert Carradine to play the Younger brothers, and Jeff and Beau Bridges, the Ford brothers.

His goal is to find a balance between the two kinds of acting -- on the stage and the screen -- while getting more involved in producing, directing, and writing. At the moment, though, he's enjoying his new stay on Broadway as the outrageously homicidal Sidney Bruhl. "There's much more enthusiasm," he said, "knowing I'm going to be going to the theater tonight and working in front of a live audience, than there would be knowing I've got to go on a set tomorrow and work in front of a camera,"