WHEN IS that play finally going to open?" is a question that's been asked more than usual this season - and sometimes by people who paid the full ticket price and then found themselves seeing the play before the reviewers.

Previews - as theater performances that precede the formal opening night are called - used to be considered a bargain. You might be signing up for an untested evening of theater, but at least the price was right.

Those days are past, however. Previews are no longer cheaper than regular performances, either here or in New York. Arena Stage does have a subscription plan that offers, far in advance, lower rates for pre-opening performances. But if you go to the Arena box office for a preview at curtain time, you'll pay the regular rate. And at the Kennedy Center, preview rates and regular rates are exactly the same.

No one has to go to a play before the critics do, of course. But previews are a way to see a play before the reviews are written, and some theatergoers are adventurous and prefer to make up their own minds. Others have learned from experience - the "Annie" experience perhaps - that tickets are hard to get once favorable word speaks. Everybody wants to see the hits, no one wants to invest in the flops.

According to Roger L. Stevens, chairman of the Kennedy Center, "If it were up to me, I'd reduce preview prices. But all prices are set by the incoming managements. New York no longer has reduced prices for previews and we must deal with them."

Recently circumstances at the Opera House and at Arena Stage postponed announced openings for "Timbuktu!" and "The Caucasian Chalk Circle," a major cast replacement in the first, technical problems in the second. What had been planned as official performances became previews; tickets already had been printed and sold. There were grumbles. Was someone being cheated?

But because theater is handcrafted, and each performance can happen only once, theater is a costly luxury. It doesn't always get "finished" on time. In fact, almost never does a work reach a stage exactly as the creators planned. Too many people, too many imponderables are involved, for a work to come to life its first time before an audience. It is unlikely to be ideal the first or even the 10th performance.

And previews actually are part of a larger system of testing and adjusting - the Tryout system. While New York success no longer is vitala to acceptance over the rest of the country, it remains the theatrical capital.Thus, there are traditional Tryout Towns, such as Washington, Boston, Philadelphia, Los Angeles, Detroit and Chicago. Production battles are won or lost in them, and previews for tryouts are part of the testing ground.

The Eisenhower's "First Monday in October," the National's "Spotlight" and "Timbuktu!" at the Opera House all had previews before their official opening as tryouts.

There's nothing especially novel about this. The European system of "repetition generale" has long been extended to friends, critics and other enemies. There are the Paris, Munich and other assorted versions of operas.

Union regulations further the need for previews because rehearsals never begin on full stages. Plays get their legs in small, tacky rooms. Neither scenery, props, stairs, doors, costumes nor lighting may be used until all are moved into a theater at considerable cost. It sounds crazy but that's how it works.

It takes hours, even days, to hang scenery, to set lights where the director and designer want them, to test whether a pink or red "gel" will show the decor and custumes to desired advantage. Even for plays which are touring, each move is into a new world, for all theaters and their stages are different in shape and size.

So, even at a touring production's peak - and especially when a work is new - theater is unpredictable. Generally it's guesswork that sets an official opening date, setting the printing schedule for tickets and programs. Holders of preview tickets get as fair a shake as is possible in the iffy theater world.

The Tryout Towns, in fact, have developed passionate guinea pigs. i know people who go to previews and then catch a show again during the end of its tryout run, when, if luck strikes, the production is "frozen."

Audiences are half of any play. It is relatively easy for scholars to sit in libraries and regret that Shakespeare had to create so milky a character as Friar Laurence to make "Romeo and Juliet" miss their meetings, but when it rolls properly, the audience accepts the happenstances caused by that bumbling friar. What about Desdemona and that handkerchief Othello gave her? How can she drop it in plain view of us all and not see it herself? It must seem accidental. Unfortunately, Shakespeare's Quartos and folios do not even hint of the changes his plays went through.

While there are no immediate plans for "First Monday in October" after its 10 weeks here, with both stars, Henry Fonda and Jane Alexander, having filming commitments, it provides an example of how a play takes it shape from audience response. When "First Monday" was tried out two years ago at the Cleveland Play House with Melvyn Douglas and Jean Arthur, there was, besides the Chief Justice, a fourth major character. Lee and Lawrence omitted him in their rewrite: "We didn't need him" is their explanation for dropping a Common Man character who, they realized watching performances, was outside the action.

"Spotlight," which vanished swiftly, must have had qualities to recommend it. When I heard part of the score played by its creators, Jerry Bresler and Lyn Duddy, at the Kennedy Center's theater party sampling, the title song certainly had zing. Who was to know then that when Gene Barry croaked it, it wouldn't?

"Timbuktu!" seems to have a future. Thanks to Borodin, the Wright-Forrest score has tunes, the fable of the Arabian storyteller has been viable since 1911 and there are four major black performers in the leads. The postponement of the official opening for a week in favor of previews seems wholly justified. Ira Hawkins was chosen to replace William Marshall in the central role and the result of his security after previews did much to please many in the first-night house.

Previews also gave Joseph Leon the chance to settle into the part Zero Mostel was to have acted in "The Merchant." As matters turned out, Mostel's New York admirers would stand for no one else, but at least Leon and the play had the chance to impress some reviewers.