HERE'S WHAT one actress does on her first night off after the first week of the first musical she's ever dond: a pair of sold-out, raucous, two-and-a-half-hour solo concerts at Constitution Hall. Happily, there is no matinee the next morning; if there had been, Patti LaBelle would have been there, too.

"I'm a crazy lady, thinking I can do everything," says the longtime pop star who recently made her theatrical debut in Vinnette Carroll's "Your Arms Too Short To Box With God" at the Warner. On a weekday afternoon, Patti LaBelle sits tentatively on the sofa of a borrowed hotel suite -- hers is too cluttered and messy with the business of being in an extended run. One senses a tremendous strength and earthy warmth in this woman. A bit of courage, too; after 20 years of nightclub dates and concerts, it's quite a switch to musical theater.

Pop stars going to the stage is old news that seems fresh when a new generation of singers treads the boards. Linda Ronstadt, Andy Gibb, David Bowie are continuing a tradition that goes back to Al Jolson. For the stars who go this route, the rewards tend to be more personal than financial -- artistic vindication, ego satisfaction, whatever.

For LaBelle, appearing in Carroll's award-winning gospel musical was a step cautiously taken. Earlier this year, she had been approached to do the music for a television version of "for colored girls who have considered suicide when the rainbow is enuf," part of a 25-week series starting on PBS in January. Through that, she landed the role of Maggie Holmes in the PBS version of Studs Terkel's "Working."

"At first I said, 'I've never done it and I don't want to do anything where I'll end up looking like a total a--,' excuse my French. There're a lot of singers who like to dance and a lot of actresses who like to sing and dance and sometimes, they just can't do it all. I tried it and it was very easy. Then they called me about this play . . . and I really didn't want to do it. I didn't want to do an all gospel show. I am a black performer; I sing, gospel, pop, rock 'n' roll, jazz, opera -- I sing anything. It seems so obvious for a black singer to sing gospel and for that reason I said no -- at first. Then I thought, that's pretty stupid." It also helped that the play was strong, that the role addressed her more as a singer than as an actress, and that author Carroll was willing to expand her role.

"After trying it at rehearsals, I started feeling very free, like it was something I should have been trying before. It didn't matter if it was gospel, it was something different for me. I'm glad I'm doing it. From this, Vinnette will write something special for me, or I'll get something else in another play that will have more to do with lines and acting."

IT'S MANY miles from Philadelphia to Washington, particularly if you travel the rock road as Patti LaBelle has. With Sarah Dash and Nona Hendryx, she started out as Patti LaBelle and the Bluebells, high school girls coming up with a sound that would reach its apex with a similar Detroit group called the Supremes. In fact, one of the early Bluebells was Cindy Birdsong, who went on to replace Florence Ballard in the Supremes.

The Bluebells' success with hit songs like "I Sold My Heart to the Junkman" and "You'll Never Walk Alone" was a prelude to an upward sweep in the '70s, when the group became LaBelle, space-age disco queens whose shows were glitter and whose records turned to gold with hits like "Voulez Vous Couchez Avec Moi?" (pardon her French, again) and "Lady Marmalade." Then, after a 17-year partnership with Hendryx and Dash, LaBelle decided to go her own way, a move that has been vindicated by the spectacular sales of her five solo albums and her strong draw on the concert circuit. Washington has always been one of her best towns -- which may be why the Warner show started selling well when LaBelle's name was announced; in fact, it's been extended to its maximum run before hitting the road to six other cities.

"How old am I, 37?" LaBelle asks, not sure whether that's a sign of energy, endurance or age. "As long as I've been performing, I've never changed; at least I don't think I have, and I don't think the public has seen a change in that 'she's gone Hollywood' or 'she's too good to do an hour show anymore.' I don't consider myself an artist who has made it, though I'm on my way. Too many artists, once they get to that plateau, they stop giving, they don't care anymore about the public."

On stage at the Warner, LaBelle sometimes feels alone. She was, she admits, apprehensive about taking to the stage without a band to back her. "I was never nervous," she points out, "I was just afraid of not liking it, not being able to discipline myself enough to do the same thing and wear the same clothes every night . . . the being so scheduled and precise. That's what I was afraid of, but I felt I could pull it off."

The disciplines of theater and concerts, LaBelle says, are not entirely dissimilar. "I can relate to a lot of people on stage because I have the same thing with my band nine musicians . I just have to remember that they're waiting for my cues, which is different than with my band. One night at the Warner my helmet fell off. . . . I also lost it on opening night into the pit . . . my shoe busted . . . but you can't stop. It didn't embarrass me, it's just that it wasn't my show. I would have stopped and talked about it, but here you just keep going because everything depends on your timing.

"I'm learning, I'm more confident in my movement; each lyric has one movement, whereas it's not like that in concert for me. That was on my mind for the first few days. Things are getting easier. I'm not as confident as I should be," she laughs, "but I feel I can make it."

One thing LaBelle's particularly happy about is that with her in it, "Your Arms Too Short To Box With God" has been attracting new audiences. "I have a heavy white following and a heavy gay following. My being in the play doesn't give them an excuse to come -- they'll come anyway -- but it's as if I'm opening those doors for people to come in and see what black gospel is all about. I looked in the audience the other night and I saw some of my fans from Constitution Hall who would probably never have come to see the show unless I was in it. Last night, there was a white lady, about 60 years old, and she was shouting more than I was. Whoever it brings, well that's what I'm all about anyway in my performance as Patti LaBelle."

Patti LaBelle gets set to run off to a television station; later in the day, she spends 20 minutes with House Speaker Tip O'Neill swapping show biz stories. Her Pennsylvania congressman, William H. Gray III, enters a testimonial to her in the Congressional Record, probably the first LaBelle record that's unplayable. "I want to be in all the history books," she says matter-of-factly. "I want my kids to say their mom was not a legend in her own mind, but a legend in her own time. I know I'm much better than a lot of the people out there and I know that I have to knock a few ladies off the top row, so I have to keep up my energy level."

She thinks for a moment and eases off a bit. "It's not that I want to knock them off. I just want to be up there with them; there's room for us all. I feel as though it's my time to make a stand on the same level. Why not me?"