Here's Bob Prosky, the Arena Stage veteran who's been likened to a clean-shaven Santa Claus. He leans back on a banister that wiggles up as far as you can see, sips Scotch, says he feels funny in his tux, and, eyes soft under brows that curl up at the ends, talks of Arena's 30th anniversary like religion:

"One of the reasons this theater has lived is that it has celebrated the moment," he begins. " . . . Oh, yeah, like every other week I've wanted to go somewhere else, but the stage is home to any actor worth his salt. Unfortunately, it doesn't pay very well. So I just finished a movie with James Caan to send my kids to college."

Washington's pioneer resident theater company celebrated three decades on Saturday night, and a curious sort of celebration it was. Or maybe realistic is the word. Because mixed in with the food, the hoopla and a performance by pianist/comedian Victor Borge came the plain facts of running a theater that, however excellent, suffers from new competition and a sick economy.

"I don't know how that problem will be solved," said Zelda Fichandler, Arena's straight-talking founder who was talking very straight over the fruit tarts and accolades. "I just don't know. We have an enormous deficit -- $900,00 . . . I worry terribly about it."

The benefit evening started democratically with Victor Borge at Arena Stage, but then became divided economically. In other words, if you paid $100 per ticket, you were to attend the party and supper afterward at the theater itself. If you paid $250, you were supposed to get in a cab, or probably your limo, and head up 16th Street for the Mexican Embassy. Ambassador Hugo Margain, honorary vice chairman of Arena, wanted to see you for a black-tie buffet supper.

Now what you saw once you got there was this amazing wooden banister that curled up endlessly to who knows where. It might have taken a 7-year-old 25 minutes to slide down it. (This was also the banister upon which Prosky would lean and ruminate later in the evening. Surrounding him were painted murals of Mexican donkeys, poppies, water jugs, palm trees, mountains and peasants.)

The guests at this gathering included arts patron David Lloyd Kreeger, the Livingston Biddles, Sen. Alan Cranston (D-Calif.), the Danish ambassador who talked Danish to the Daanish Borge, and people from corporations who had bought blocks of tickets. Most of them don't regularly hang out in embassies.

"I wouldn't want to do the housework here," observed Rita Andrews, who does public relations for C & P Telephone. She was wandering around with C & P cohort Dave Cook in a mamoth room that had blue tile, high ceilings and a round table big enough to hold a rhinoceros. But neither of them could figure out just what it was used for.

"Probably just a breakfast nook," surmised Cook.

Borge, who was the star guest, had scarecly arrived before he was shuttled over for pictures with Fichandler, Cranston, Prosky, and the Mexican ambassador. The photographers clicked on and on and on, some standing with their backs right up against the banister over which you could see the cold, hard floor below.

"Why don't you take two steps back?" Borge suggested to them amiably.

Pretty soon, everybody went home. And pretty early too, this being a fairy starchy embassy function that didn't lend itself to raucous behavior far into the night.

But at the party at Arena Stage, things were different. A rehearsal room had been transformed into a disco, and people were hopping all over the place. A house carpenter danced with a house manager and Mark Olsen, an actor from "Mummenschanz" at the Kennedy Center, wore a crushed silver polyester jumpsuit he said was inflatable but hopefully not flammable. No starch at all.

Then there was Tom Aldridge, another house manager. During a dancing break, he reminisced about the great end-of-the-Arena-season party of 1978-79. b

"People were dancing or getting loaded until dawn," he said wistfully. "And people were actually found in the closets and back rooms for the next day, or asleep in the aisles. It was sort of like the ruins of Pompeii, with bodies lying all over the place."

Fichandler eventually made it over to the Arena Stage after the Mexican Embassy supper. She took a quick look at the disco, then wandered downstairs to where a jazz band was playing in the Old Vat room.

There, during the band's break, she took a seat alone at a table. She lit a cigarette in the dark, looked out toward a spot of light on the stage.And smoked, quietly.

Her thoughts, she said, were "very private."