Now that thousands of folks have seen "Singin' in the Rain" at the Paper Mill Playhouse in Millburn, they have been impressed, no doubt, with Michael Gruber's sloshing around the stage in his big number.

Indeed, applauding audiences well may ask: "How does he do that?" Well, he has a lot of spunk, that's for sure, and skill, too. He boasts both with gusto. But he's quick to commend his supporting players who, in this instance, seldom are seen but play pivotal roles behind the scenes. Without them, Gruber wouldn't be all wet.

Eight times weekly, five minutes at a stretch, Gruber performs the show's title tune and, by golly, he does sing and dance in the rain while wearing a suit, doffing a hat and waving an umbrella, more of a prop than a protection. At Paper Mill's insistence, when it rains, it pours. "If you don't have that number, you have problems," says Bob Murphy, supervisor of the theater's construction shop in Edison. "Talk about a signature tune!"

Two months before the show opened, Paper Mill's behind-the-scenes wizards went to work creating and crafting riveting rain. Confident and consistent, Murphy's key concern is safety. Technical rehearsals are crucial.

Gruber was a good sport, recalls Murphy, even when initially icy H2O drenched the actor. "That's a lot of cold water to dump on someone!" quips Murphy. The unintentional mishap inspired a heating system. Now the water is warm.

"It's like taking a shower," reveals a happy Gruber. He points out actors' contracts often specify the water must be warm so they don't get sick. So far the young, healthy Gruber reflects another of the musical's songs. He's "Fit as a Fiddle." Right after singing and dancing in the rain, he runs off stage, tears off his sopping wet clothes ("I'm soaked"), dons a robe, wraps towels around his throat to coddle his tender tenor vocal chords, and with haste drys his hair. He has only 15 minutes to effect the change.

Paper Mill audiences are accustomed to awe when it comes to the playhouse's powerful productions. "Singin' in the Rain" continues to be a sunny show, even after a six-week run. It's in the home stretch now, its stage strengths unabated, as it heads with a flourish toward the finish line next Sunday. Once the-powers-that-be at Paper Mill decided to go into the rain business, Murphy's crew had its work cut out for it. An elaborate and effective rain making system is the result.

A "rain deck," actually a big wagon, rolls down stage and rests at a pitched angle, allowing water to run into a huge holding pit. "There is more than one type of rain," comments Murphy. Overhead there are three water pipes, two spraying "a nice, fine, spring rain," notes Murphy, and the other producing a drenching downpour. "The director (James Rocco) really wanted a pounding effect," Murphy remarks.

Hundreds of gallons of water are pumped and recycled as a pair of 10,000 watt heaters keep it comfortable for the singing, dancing, splashing Gruber. Off stage it takes two crew members to assure everything is right as rain. Bob Durish is the "rain maker" and Brian Pollock is the cuer.

Engineering is emphasized, but artistry is uppermost. Timothy Hunter is the designer of lovely lighting that evokes nature, and Paper Mill's touted technicians bring it all to life.

"It's lit beautifully," says Gruber. "The number is so wonderful technically, all I have to do is go through my paces."

That's not as easy as the genial Gruber makes it sound. Most dancers would pale at walking on a wet floor. But Gruber explains the rain deck surface has a purposeful pebbly finish, something like gritty sand crossed with adhesive shellac. "There's a lot of traction there," avers the actor. "It's not like dancing on linoleum."

To dance and to sing memorably in the rain, Gruber even wears a water repellent microphone. Precise science combines with show business flair to make a few magical moments on stage.

Each time Gruber does the notable number, it's a bow to Gene Kelly, creator of the Don Lockwood role in the classic movie. Gruber calls Kelly "the master" and admits the stage number follows the film solo nearly "step by step." But the actor, between rain drops and downpours, does his best to make a splash.

"I'd like it to be a bit of my own," he says. "It's a joyous experience."

For the Paper Mill personnel it's all in a day's work. On sunny summer days in Edison, shop workers practiced pouring rain off the building's roof. Inside various set components originated, from poster paintings to rococo art objects.

Back in Millburn no executive was uninvolved in another opening, another show. Roy Miller, an actor turned administrator, assumed another role as a filmmaker. Prior to Paper Mill, he made videos for Pepsi.

Miller offered his expertise to shoot some silent film footage for the stage show. He persuaded Angelo Del Rossi, Paper Mill's producer, to act a cameo role in the celluloid caper.

"It was shot in a day," says Miller. Then 21 hours of editing later, plus transposing color to a grainy black and white for an authentic silent film look, resulted in some fine fun on stage.

In a reflection of Paper Mill policy and practice, Miller asserts: "We took on the challenge ourselves and we had a great time."