IT'S 7 o'clock, the ropes are down, the "house is open." For the next half-hour I'll see everyone from congressmen to would-be crashers.

Actors say no two audiences are the same. "How true," echo the ushers.

It is well-known that most people who usher do so because they get to see the shows free. But that is only part of the reward. For although the events onstage may be repeated, what goes on out front varies with each performance. So from the moment I put on the red jacket that identifies the Kennedy Center usher, I know my evening's activities at the Eisenhower Theater will never be dull.

As the patrons enter, I am happy to spot the regulars. They know their way around, and they are usually as glad to see our familiar faces as we are theirs.

Organized groups are easy to identify. A convention usually means a cheerful audience. (On occasion, a precurtain cocktail party has been known to make these people a little too cheerful.)

Sometimes there are visitors who decide to get into the spirit of the play by dressing like the lead character and parading around the lobby. Sherlock Holmes brought out a slew of deerstalker hats and houndstooth coats. Dracula and Superman capes abounded at those shows.

Whatever the composition of the audience, there will be the usual number of patrons without tickets. Most often the tickets will have been mislaid or forgotten, but other explanations range from lamentable (stolen wallets or purses) to mildly, or even wildly, funny. Many tickets fail to survive the washing machine, although some make it, pale and limp. Dogs, cats and, now and then, a baby find them good eating.

Once, both members of a divorcing couple thought they were entitled to their Theater Guild subscription. Each arrived on the appointed night with a date. He had the tickets and was seated when she arrived. She had held the subscription before their marriage and had not dreamed he would be there. I, having a full house but not the wisdom of Solomon, had a dilemma. The gods smiled, however; someone gave me two unused tickets.

Seeing a grown man walk in one night clutching his teddy bear warned me to be ready for something. An usher soon rushed out to tell me that the man was in the front row and had put the bear on the stage, facing forward, presumably so he, too, could enjoy the play. "Long Day's Journey Into Night" didn't really seem to be bear fare and the prospect of having the leading players walk onstage and see him sitting there was unnerving. Happily, the owner agreed to let the bear watch from his arms.

The presence of Secret Service agents signals the advent of a high official. "Who's coming? Is it the president?" Sometimes it is, but the arrival of almost any dignitary sends a flurry through staff and audience.

It's fun to observe that a star will be as impressed by a high government official as the official is by the star. This was never more noticeable than when the Supreme Court members came to see themselves being portrayed by actors in "First Monday in October." A mutual admiration society evolved.

The only adverse moments come when someone is stricken with a sudden illness or accident. The nurse, police and others rally to help. Recently, Michael Cassidy, a stagehand, was vital in reviving a woman who had suffered a heart attack just before entering the theater. Fortunately, such calamities are few.

Ushering attracts a variety of people--students, teachers, housewives, retirees, people in managerial positions or clerical jobs. An almost spontaneous feeling of fraternity erases wide variations in age and background because there is one common denominator. We do it for love more than for money. And . . . who knows what may happen tonight?