It may be very well for people in the movies to jauntily tell Great Stars to "break a leg" when they go out on stage, but what happens in real life when the poor wretch really does?

What happened, for instance, when Henry Fonda had a heart attack right in the middle of the run of "Darrow" and had to miss all kinds of performances? When Bette Davis' nerves decided to pinch during rehearsals of "Miss Moffat?" Or when George C. Scott got the flu during "Sly Fox" and simply could not tred the old boards?

And what happens in the case of tragedy, as when Gertrude Lawrence died while starring in "The King and I," or, more recently, when Zero Mostel died unexpectedly after one preview performance of "The Merchant," due to open at the Kennedy Center on Sept. 28?

In theory, at least, insurance exists that can cover these contingencies and more, and in the cases of Fonda, Davis ans Scott, it did. But many times, as with Mostel, this is only in theory, because theatrical insurance is costly and difficult to obtain, an exotic beast with a will very much its own.

These policies are so hand-crafted, each one is different; the companies evaluate, they give you a price and either you pay it or you don't get insurance," explains veteran producer Gene Wolsk. Robert Boyar, head of an insurance brokerage firm that specializes in the performing arts, adds that there are simply "no rules on the coverage of a star. There is nothing cut and dried in this business."

Because of the risk involved, only two American insurance companies, Fireman's Fund and Chubb and Son, will even write this type of theatrical insurance. And since the cost of the premiums can run to $50,000 and higher, it follows that only the real heavy hitters, the stars whose absence would mean enormous refunds or even cancellation, are even considered for it.

Even here, differences exist. A spokesman for producer Alexander Cohen, for example, called insuring stars "expensive but an essential precaution" for shows like Liv Ullmann in "Anna Christie" and Peter Cook and Dudley Moore in "Good Evening." On the other hand, Harold Prince, producer of "Follies," "Candide," and "A Little Night Music," is known to eschew insurance, preferring shows that are built around groups of people as opposed to being carried on the not always sturdy shoulders of a major performer.

Actually, there are two types of star insurance, not one. The cheapest and easiest to get is called abandonment: its premium rate is only 1 or 2 per cent of the value of the policy, for instance $10,000 to $20,000 on million-dollar coverage.

"Let's say you're going into rehearsal in September but you've signed a star in March," theorizes producer Wolsk. "You start raising money and possibly spending it on the basis of that star. You have either paid for or committed to the set, the costumes, the guarantees to out-of-town and New York theaters, ads, rehearsal and directors fees. The your star gets seriously ill before you even have a single performance." You can then abandon the show - which means agreeing not to do it in any form for from one to three years - and collect for your losses.

However, as insurance man Boyar points out, abandonment is not a certainty."It must be recognized that a play worthy of all that it takes to be produced is not so easily come by. And no money is made in abandoning a venture, nothing is gained, you're back to square one with a loss of all potential profit. So a producer would hope that he could replace the star."

The second type of insurance, non-appearance, is at one simpler to understand and more expensive. Your star gets sick and misses a performance. You go with the understudy and pay a huge amount of refunds, or you cancel the performance but still have to pay the rest of the cast, the theater owner and so on.

Non-appearance insurance can help - an average policy migh repay $10,000 per performance with a weekly limit of $80,000 and an aggregate value of anywhere from a quarter of a million to a million dollars - but it will cost. There is a deductible which says the first two missed days, or perhaps more, don't count and the premium rate can be as high as 6 per cent, which means $60,000 on a million dollar policy.

The fascinating and quirky thing about star insurance, however, is that no matter how you write it, it doesn't always cover all the contingencies. Take, for instnace, the case of the musical "Chicago."

What made "Chicago" special right from the start was that the producers chose to insure director Bob Fosse as if he were a star. And, in fact, Fosse had a pre-production heart attack and was told by his doctor he had to take a 6- to 8-week rest. The producers had a legitimate case for abandonment, but they didn't want to abandon. Instead, an arrangement was worked out by which the insurance company advanced the producers enough money to keep the project together until Fosse recovered, and "Chicago" went on to become a major hit.

With Zero Mostel and "The Merchant," things were more complex still. According to Bernard Jacobs, president of the Shubert Organization, one of the play's coproducers, "non-appearance was out of the question. Since Zero had missed performances in the past, the insurance companies wanted a high number of deductible performances and in atrocious premium rate. Plus he probably would have refused to take a physical and he could have been insured without one."

As to abandonment insurance, Jacobs says, "It was never contemplated. We always felt we had to go forward with this play. We knew we could not abandon this project. It's one of the best plays written in many, many years, and we hope it will be an enormous success."

Leon Jacobs, Mostel's understudy, has taken over the lead role. Most of the advance ticket sales before Mostel's death were to Theater Guild subscribers, and the Kennedy Center reports that there have been almost no requests for refunds.

As to the future of star insurance, Bernard Jacobs is not terribly sanguine. "It's most difficult," he says. "Even though we did not have insurance this time, the companies are aware of the risks they didn't take as well as the ones they did. Each time you collect money it makes it that much more difficult to write that kind of policy again."