It is curious, but true, that most actors love to play characters who are wicked or deformed. Assembling a wardrobe of limps, sneers, ominous glowers and sly smiles, the player then mulls over the nature and uses of evil, investigating the dark side of the soul.

Shakespeare's "Richard III" is a perennial favorite in this category, and Stacy Keach is no exception to the actor's rule. Whatever the earthly inconveniences of undertaking the role at the Shakespeare Theatre at the Folger (where the play opens Sunday) -- three months away from his Malibu home, a pittance for a salary, the quantity of words to learn, weight to lose and muscles to coerce -- it is clear the role is a tonic for him.

"It's the agony and the ecstasy," he said. "The agony is when you're not there, when it isn't right... ." The ecstasy, evidently, is when it is.

Laurence Olivier modeled his fabled interpretation of the power-greedy king physically on the American director Jed Harris, a loathed figure under whom the actor had suffered, and who was reportedly also the inspiration for Walt Disney's cartoon version of "The Big Bad Wolf." Rip Torn did a production in the '60s in which his Richard III was Richard Nixon, an interpretation that was as irresistible (first act: ascendance to power, second act: paranoia) as it was flawed.

Keach does not cite a specific person for his model, but says he has longed to play the part for years. Financially cushioned by his work in television, Keach has finally realized an oft-delayed plan to tackle the role within the classic confines of the Folger. It is his first major stage role since 1986, when he starred in a production of "Idiot's Delight" at the Kennedy Center as part of the ill-fated American National Theater.

For nearly a month, Keach has reported for rehearsals to the Folger's tatty rehearsal hall in the back of a former Safeway on Seventh Street SE. Recently, the company moved onto the Folger stage, the better to gauge the levels of sight and sound and the swing of mace and sword.

At a recent rehearsal Keach wore a high-necked black shirt, which covered a one-shouldered foam hump, a cross hung round his neck, and a black glove over the "wither'd shrub" that is the king's hand. It was only later that one realized this king was also wearing khaki shorts and jogging shoes. His hair is thin and graying (not the curly dark hair of his official photo), his skin freckled, and his voice, as ever, the rich honey-and-sandpaper instrument that any actor would envy.

Later, while other subjects seemed not to ignite his interest, Keach was happy to discuss the conceptual and physical roots of his Richard's deformity. "I started out thinking it was polio, then cerebral palsy," he said, leaping up in a nearly deserted restaurant to demonstrate a lurching, twisted walk that seems sadly familiar. "But I abandoned modern diagnoses. I just know exactly what it is and what it will look like."

There will be a leg brace (Keach has almost no cartilage in one leg, thanks to high school and college football) and a built-up boot, the hump and a prosthetic hand. And, as a final touch, he plans to shave his mustache to emphasize the scar that knifes his lip, the legacy of a harelip operated on in childhood.

"You might as well go with what you've got," he said, with a grin.

All this is within the tradition for Richard IIIs, despite recent scholarship that suggests Shakespeare's portrayal of the king was based on propaganda, that Richard was not deformed at all, but depicted as such as part of "the outrageously imaginative fruit of Tudor chroniclers' desperate need to rewrite an embarrassing and reprehensible series of events," according to the Folger's dramaturg, Liza Henderson. "In fact it seems that Richard was a king of some quiet but extraordinary qualities."

The local president of a society created in 1924 to clear the king's reputation said he spoke to Keach at a Folger party. "I asked him to go easy on my guy," said Anthony Collins, who represents the Fellowship of the White Boar here. "But it's Shakespeare who is really our problem. They have to do it as he wrote it."

"I told him, 'Hey, we're not doing a documentary here,' " said Keach. "A Richard III without a limp, a hump, a twisted grin? No way."

In fact, director Michael Kahn has augmented Richard's opening speech, the famous "this is the winter of our discontent," with a few lines from "Henry VI, Part 3," in which the same Richard describes his deformity:

Why, love forswore me in my mother's womb:

And, for I should not deal in her soft laws,

She did corrupt frail nature with some bribe,

To shrink mine arm up like a wither'd shrub; To make an envious mountain on my back,

Where sits deformity to mock my body;

To shape my legs of an unequal size;

To disproportion me in every part,

Like to a chaos, or an unlick'd bear-whelp

That carries no impression in the dam... .

But enough about the physical stuff. "I am most concerned about the passion," Keach said. "The duplicity. Richard's quest for power and obsession with success stems from his deformity... . He takes such delight in being bad."

Stacy Keach has been described as one of this country's classically trained actors, considered in his youth to have been primed to inherit the cloak of John Barrymore, Edwin Booth, Alfred Lunt or whichever theatrical god one wished to invoke. So what is one to make of his career, starting as it did with Shakespeare, Ibsen, a slightly scandalous (at the time) off-Broadway political satire called "MacBird," then adding small parts in odd but respectable movies, and then sliding into television?

In 1972 he played both Hamlet and Martin Luther onstage; in 1983 he did "Princess Daisy" and two Mike Hammer movies for television. "Keach was one of the hottest young actors in Hollywood," says his entry in the International Directory of Films and Filmmakers. "Then in what is one of the sharpest reverses in movie history, Keach's film career sank in the 1970s after a string of unsuccessful, forgettable films ('Gray Lady Down,' 'The Dion Brothers,' 'The Squeeze'). He finally hit the cinematic bottom when he played second fiddle to Cheech and Chong in 'Up in Smoke,' and Pia Zadora in 'Butterfly.'

"Ultimately, Stacy Keach's versatility and range may have undercut his screen success," this particular historian concludes. "Taken as a whole, the very diversity of his acting career clouds any singular impression left by his movie roles."

"I think that was truer at a certain stage in his career than it is now," said director Arvin Brown, who went to Yale Drama School with Keach and has worked with him since. "Stacy was blessed and cursed with the best technical equipment of any actor in our class. He had abilities more common among English actors than American. And because he was young, and his life experiences were not equal to his abilities, he sometimes appeared facile... . He is not a personality actor, not a super-stud. That type of thing never interested him. Superstars tend to be people whose personalities are easily graspable. He's much more interested in submerging himself in a character."

At 49, Keach has certainly had some life experiences, including a six-month stint in an English jail for cocaine smuggling that he now prefers not to talk about. At the time -- 1985 -- he performed the public expiation society seems to require of its famous sinners, appearing at a congressional hearing, narrating a documentary about drug abuse in the workplace, and discussing his rehabilitation. The producer of "Mickey Spillane's Mike Hammer," Jay Bernstein, even traveled to 15 cities while Keach was in jail, as part of a personal campaign to protect Keach's reputation. A person who has paid his debt to society should be allowed to get on with his life, Bernstein argued.

And it would seem Keach has. There have been two Mike Hammer television movies filmed since then (the most recent in 1989), and after a slight pause other television work started coming, including such things as hosting the Thanksgiving Day Parade and joining the 14th Annual Circus of the Stars. He is in a new movie, "Class of 1999," and in 1988 played Ernest Hemingway in an eight-part series.

"He has had a lot of personal stress," acknowledged Brown. "I think it has enriched his acting immeasurably. My wife, who is an actress, likes to say that acting is the only profession where you can use your pain."

Brown recalled that his first professional job was with Keach, although he didn't realize it at the time. After his year at Yale, Keach put together a one-man program of Shakespeare and Brown directed it. "He spent the summer touring around with this thing, and when he came back he handed me $50 or $75. I was stunned!"

The two plan to work together again next year, with Brown directing Keach in a one-man show about John Barrymore, written by William Luce. Barrymore is a figure who has always intrigued Keach, who also comes from a theatrical family. His father, Stacy Keach Sr., was a character actor and producer, and his brother James is also an actor (the brothers collaborated on a movie called "The Long Riders" in 1979). His third wife, Malgosia Tomassi, is an actress Keach met in England. They have a 2-year-old son, Shannon, who is here with them.

There may be some of "Richard III" in the Barrymore piece, Keach said, played as Barrymore would have. At that point he drew back his lip and snarled in melodramatically villainous fashion, in the old-time bombastic style. It was a perfect imitation.

"Can't do it like that anymore," he said. He sounded almost mournful.