INCREASINGLY, actors and actreasses are risking the challenge of solo vehicles.

Though it's denied, and sometimes with reason, economics does account for some of this. But there's far more to it: It takes a star performer's total immersion in exceptional material to succeed.

Having already succeeded with Will Rogers and Harry Truman, James Whitmore is honning his long-considered Teddy Roosevelt in the National's "Bully," set for a long tour.

Hal Holbfook is about to return his continually changing Mark Twain to New York for a dozen performances.

Emlyn Williams this season turned to Dylan Thomas after a Kreeger visit as Charles Dickens.

Eugenia Rawls tours in a string of three entertainments based on Tallulah Bankhead, Fanny Kemble and a score of Women of the West. Billy Dee Williams tackled Martin Luther King Jr., Eileen Heckart limned Eleanor Roosevelt and Julie Harris triumphed as Emily Dickinson. TV's Carl Betz has begun a tour as Gen, MacArthur in "I Shall Return," and Fritz Weaver had a recent go at Saul Levitt's "Lincoln."

To this plethora of stars already appearing in one-performer scripts add Cheryl Crawford's portrait-from-her-own-words of Janet Flanner, The New Yoker's Genet, For 40-odd years she has written the magazine's Letter From Paris. Madeleine Renaud soon will be playing this in Paris and Grete Mossheim will be Genet for Berlin. Helen Hayes considered Crawford's script but is sticking by her stage retirement. Instead, Barbara Bel Geddes will be the Ameraican in Paris for American audiences.

Ans before the shows of this past year, William Windom had become James Thurber, Paul Shyre was H.L. Mencken and Dorothy Stickney was Edna St. Vincent Millay. Cornelia Otis Skinner chose famous women, the wives of Henry II, as well as a solo play, "Mansion on the Hudson." The astonishing, revered Ruth Draper managed to score as anonymous women of many nations and achieved a true "monodrama" in "Three Women and Mr. Clifford."

Playwright Jerome Alden titles Whitmore's vehicle "Bully," an exclamation Theodore Roosevelt made a national catchword during his presidency. Though he scored hugely as Rogers and Truman, Whitman says Teddy Roosevelt is closest to his heart:

"My mother brought us up on Teddy Roosevelt and that can be said for many children of my generation. From his championing of the vigorous life to the little incident about Teddy was hero who sparked pride in America till the day he died. It's this I try to catch in my potrait, the excitement and admiration he created, the dream he renewed for his country at the start of this century. I want to convey this to young people.

"These plays, I think of them as plays, must, above all, be entertaining. The minute they bore an audience, the figure becomes unreal. This does make it hard, sometimes, to state exactly what they did. Rather than explain the byzantine intrigues of the Panama Canal's origin, my Teddy just shouts: "I TOOK Panama."

"I'm still feeling my way. So far I've only done him 10 times. Another reason I always wanted to play Teddy is that I look like him. I didn't look like Rogers or Truman, though I'm glad you think so. But with the mustache and the added glasses, my face does resemble Teddy's. And, for once in a role, my bowl legs are right.

"Bully," like "The Belle of Amherst," uses elaborate setting and lighting, a trend away from the Holbrook-Williams-Rawls school, though a return to the settings and elaborate costumes of Skinner.

"So,"Whitmore continues "it's not a matter of economics. When I was here last summer as Oliver Wendell Holmes in "The Magnificient Yankee," we had a score of actors and the budget wasn't much more. Sets and lighting are expensive to mount and move. 'Bully' I think of as a play with characters I talk to and the whole setup is as elaborate as many plays."

Holbrook's "Mark Twain Tonight!" is the opposite. He and stage manager Bennett Thompson are the whole company and literally pack up their equipment themselves. On their first Kennedy Center visit they couldn't get a cab on "get-away night" and at 1.30 a.m. "borrowed" two shopping carts from the Watergate Safeway to trundle their boxes to Guest Quarters. Last weekend they splurged and hired a limo.

"That's the way, do-it-yourself. I began as Twain on the 7 a.m. school circuits of Oklahoma and Texas," chuckles Holbrook, who limits his Twain to about 25 performances between January and April every year. Thompson, his invisible associate for 11 years, blocks out those months from his work with the Chicago Civic Opera.

"It's both a godsend and a killer," says Holbrook. "It makes me independent of taking parts I don't want, terrific freedom for an actor. But when I'm doing Twain, I don't do one more thing. I'm in the theater by 4 o'clock, shuffle around, take three hours for midnight. The next day I don't go out, just mull about the coming performance."

LIke Whitmore, Holbrook has learned all he can about his character, which is a lot since he's been working on the character for over 20 years. A comment is made about Twain's thoughts on women and Holbrook swiftly corrects it: "That's out of context," he explains. "Twain did write that, but it's really a part of this" and he qotes about three minutes of Twain that he's never used on stage. He thinks about what he's heard himself say. "I might use that someday, I just might."

I do vary the material. There's so much of it. I usually stick to one-night stands so as not to outwear my welcome. I'm surprised to find myself doing five nights in Washington. I once did 12 weeks with matinees in New York, but never again. So this return to New York I'm limiting to 12 performances.

"I've learned to pace my life, to enjoy my 6-year-old daughter on the Coast. When I was starting out I was damned unfair to my two sons and I always will regret that. Now I've just signed for a promising TV special, to play the Stage Manager in Our Town.!"

All the successful solo performers share the experience of living with their subjects. Williams "heard" the dramatist in his fellow Welshman, poet Dylan Thomas. From her studies in Paris came Skinner's "Paris '90" and from playing a Thurber character on TV, Windom developed his Turber portrait. Rawls acted bankhead's daughter in "The Little Foxes," the star became godmother to her children and "Tallulah" was a natural.

Not all solo productions work out as they have for Holbrook.

"The humor," claims Whitmore, "has to be there. I used to think about trying Walt Whitman, but you know, there's not a spark of humor in the man. I love his work, admire him profoundly. But he's not got a shred of humor."

Nor did Eleanor Rossevelt. A tyro playwright's effort to give the great First Lady a lot of laugh lines ultimately defeated Eileen Heckart's touching try with "Eleanor." On the other hand, only humor is not enough. A few years ago a forgotten stab at Dorothy Parker was a quick failure in London.

Like solo singers who must have perfectly turned pianos, solo actors must find miraculously turned instruments.