Theater life does not begin when the lights go on. Yet here it is, broad daylight - well, as much daylight as the East Coast presently is seeing - and four casts are working the 9 to 5 shift, anathema to the after-dark set. It's rehearsal time for three new productions, the Eisenhower-bound "The Mighty Gents," Bob Fosse's "Dancin'," Stuart Ostrow's "Stages" - and "The Act," which has been playing since June, but to which Liza Minnelli has just returned after a two-week about with the flu.

Rehearsal space isn't at all what the imagination might conjure. Performances generally take shape in nooks and crannies all over town, since union regulations decree that if a theate r is used for practice, wages for stage crews must be paid. This can add thousands of dollars to a week's rehearsal budget.

Over the years I've watched what became a couple of huge musical hits gather for first rehearsals in a tacky old hall on upper Broadway, and also witnessed several famous dramatic stars sitting around a table for a first reading above a kosher delicatessen on lower Second Avenue. "The Act"

The most extraordinary of these current practice sessions is in a theater - the Majestic - where stars Minnelli and Barry Nelson, writer Fred Ebb and all the cast are working on stage with Stanley Lebowsky at a piano instead of his 26-piece stage orchestra. The Las Vegas night club set is in use and the crew in attendance. You almost never find all this during a run.

Minnelli's flu closed "The Act" for two weeks, but it has resumed and tonight's performance will be as it has been since its Oct. 29 New York bow. This rehearsal has been called to ease Minnelli out of some of her complex dance turns. She'll seem to be doing as much, but the aim is to ration her for the eight weekly performances.

The master of this, Gower Champion, has flown in from California only to make seamless changes on the staging he took over, uncredited, for the initial director, Martin Scorsese.

Long a performer before his choreographic and directing career, Champion is aware of the energy it takes to sing and dance at the same time. There have been complaints that during some of her dance numbers Minnelli "lip-synched" to previously recorded tape. Less dancing will obviate some of that and also save the star's energy. When Carol Channing offered to do more dancing in the original "Hello, Dolly!", director Champion held her back: "You can't do that much eight times a week. No, one can." So now his advice on how to conserve Minnelli is being heeded.

Now he and the dance captain have ways for the seven Ron Lewis dancers to cover some sections of Minnelli's dancing acting. They're working on a number called "Turning" and by shifting complex patterns of poles and steps, a new exit and reentrance are created for Minnelli.

Liza-with-a-Z is fascinating to watch in her black rehearsal suit. Concentrating on what she's going to do - turn about on a pole - she looks like an underfed Miss Nobody. Turning on that elusive quality that makes a star, Minnelli, even in the sharp rehearsal lights, suddenly becomes larger than life, singing out Ebb's words and Kander's tune with arresting precision. In an instant she makes that unique transition a privilege to watch.

It will be a week or so before the new moves are brought into performance. Meantime dancers and principals at performance time will have to forget all about the new moves Tricky. "The Mighty Gents"

The atmosphere in the Minskoff Theater building, which replaced the storied Astor Hotel on Times Square five years ago, is also a surprise. In this skyscraper are spaces unfamiliar to New York theater people - shiny, clean spacious practice rooms clustered, like TV studios, off corridors leading to soundproofed doors.

Mirrored walls rise from clean floors, windows look out on the sky and The New York Times, where the future of these weeks of work ultimately may be decreed. It is strange to take an elevator to such quarters, for usually you go shank's mare up rickety stairs.

On the first day of rehearsals for "The Mighty Gents," opening its six-week run in the Eisenhower March 7, a private theatrical ritual takes place. The Equity cast elects one of its members as official "Equity Representative" to insure that regulations about hours, food breaks and general conditions are properly observed. That over, author, director, east, understudies and producer gather for official photographs. Then the door will be closed and the players free to begin their private probings.

This, all actors will tell you, is their favorite working period. Now discoveries will be made about the play, what it's really about, how the characters relate. It also can be a time of rude jolts: An actor who seemed right a casting may not progress one millimeter in deepening the character; the jell for ensemble - and voices - may not fix as the director has envisioned. Above all, this is a private time for an art that doesn't come alive until it faces its public audience.

A dozen years ago author Richard Wesley, the playwright, was a student in Howard University's drama department. Now the careers of a noted director, Harold Scott, a dozen black players - some are understudies for the eight-character play - and hundreds of thousands of dollars are riding on Wesley's perceptions.

Wesley, 31, wrote the play when he was 21. It has grown from a one-act to full-length. It's in a far more serious vein than the two vastly popular films that make Wesley a bankable writer, "Uptown Saturday Night" and "Let's Do It Again," which constarred Sidney Poitier and Bill Cosby. The Script, which decidedly has its laughs, begins with a dedication: "In memory of all the Brothers and Sisters who saw the light at the end of the tunnel . . . and chose the darkness instead."

Shedding the new snow suit he's just bought ("I was born in Newark, but cold weather is not my natural habitat"), Wesley remarks: "I'm 10 years older. So are the characters. The play's perceptions of a street gang grown older are more valid than when they were in their 20s.

"The play's title has changed from 'The Last Street Play' to "The Mighty Gents,' which is what the gang called itself in what all now see as the good old days. Their prime. The question of what's to become of them is more pressing."

Looking around the table at these largely unknown players, one is surprised how familiar many of them are.

They've been on TV, played roles at Arena, the Folger, the National, the Eisenhower. Some have worked together before, some not. All their resumes list scores of roles.

But now, it's reading-around-the-table time.

"We'll feel it when we're ready to get on our feet," says director Scott, who has been artistic director of Cincinnati's Playhouse in the Park. From Seattle to the east he's directed black plays and such traditional white ones as "A Delicate Balance," "The Glass Menagerie" and "Long Day's Journey into Night."

Dorian Harewood, whose Calyle scored in the original production of "Streamers," is familiar through TV and film. By coincidence, another member of the cast is Brent Jennings, who played Carlyle for Arena Stage and was in the Manhattan Theater Club production in his current part.

Jennings and Wesley remark on how the play has evolved since it was introduced in the O'Neill's pattern of two-day rehearsal periods for director and actors resulted in a work that still held and had a surprise punch. Vinette Carroll's Urban Arts Corps gave it a production last February that led to a fuller one in May at the Manhattan Theater Club.

Here the New York reviewers took a part in the play's future. "I cannot think of a better of more powerful play I've seen lately," reported Martin Gottfried. Mel Gussow saw it as "tautly constructed and filled with the grittiness and lyricism of the urban landscape." Douglas Watt found it a "sharply defined and altogether grand melodrama."

These economiums caught the eye of James Lipton, whose producing firm is a subsidiary of Gulf and Western and who, a few months earlier, had produced "The New Spirit Inaugural Concert" for President Carter at the Kennedy Center Opera House for a TV audience. Writer, actor, director, novelist, Lipton claims to have had little trouble getting backers for "a full-scale Broadway production."

As the day winds down, Starletta DuPois, the play's only female performer, talks with conviction of how the reading has begun to reveal the subtext.

"This is why I find rehearsals the most stimulating time. I say something, Dorian answers me in his character and suddenly something about our relationship is clearer.

"I went from a small role to a major one in the course of my two seasons with 'What the Wine Sellers Buy,' acting both roles on the two National visits, and the more you feel what's between the lines, what's not said, that is when the relationships come alive."

"It's been a good day," she says, and now, living alone, she's glad to be going home to no distractions and concentration on tomorrow morning's session when she comes to her major scene for the first time. "Dancin"'

In the honeycomb of alleys between 44th and 45th Streets, it's musical chairs time. "Sly Fox" moves from the Broadhurst to make way for "Dancin'." Neil Simon's "Chapter II" doesn't need the Imperial stage on nonmatinee days.

Both theaters are part of the Shubert Organization chain of 18 1/2 theaters. (The Shubert firm and Irving Berlin share ownership of the Music Box.)

Thus, Bernard Jacobs, the Shubert honcho, has allowed Bob Fosse his favorite Imperial stage for "Dancin" run-throughts.

In "Dancin"' Tony-Oscar-Emmy-winning choreographer Fosse is creating something new. From music scattered over several centuries, Fosse has devised a nonverbal revue of pure dance. The cast has just completed its final stage time before heading to Boston, tons of snow and an opening performance on the 14th. "Stages"

At Studio 48, on West 48th Street, the cast of "Stages" is breaking up for the day after rehearsing since 9 a.m.

"Stages" is bound to set some kind of record. Stuart Ostrow, producer of "1776" and "Pippin," is presenting a new play by an unknown playwright, Stuart Ostrow. "If I don't believe in my own play," asks the successful producer, "who will?"

"Stages" is about an actor, to be played by star Jack Warden. In the cast are some names well known to Washington audiences: Philip Bosco, Max Wright, Roy Brocksmith, as well as Tom Aldredge, Lois Smith, Diana Davilla and Rick Petrucelli.

Initially, this was to have tried out at Washington's National, but the production devised by director Richard Foreman, who staged Lincoln Center's "Three Penny Opera," will be far too heavy to travel. So, after a few previews, it will open at the Belasco March 19.

Ostrow's play baffles some of the actors, but almost have worked for Foreman before and gain confidence as, around the table, he describes the five sets and hundreds of props that will whirl around their characterizations. Foreman will clarify what is happening to Warden's actor at the variously depicted "Stages" of his life.

During this period actors have a saying: "By the fifth day, the bad eggs in the cast usually begin to smell." That's the day Equity regulations make it possible to fire actors without paying full salaries on those run-of-the-play contracts. Make it past then and if they fire you, you can settle on a solid sum or take a chance on being paid weekly so longs as the epic runs.

Four productions - each struggling to find its way on stage, and to stay there awhile. When will the job be done? It never will. Rehearsals stop, runs end, and playwights and their players come and go; but the play itself is never "finished". While playing "I Know My Love," here in Washington, the Lunts introduced a small change in Act II. It worked. It was the last performance of a three-year run.