A death invariably reminds us of the value of life, and a death in the theater can serve, once grief has subsided, to remind us of the glory of the art form.

Whendeath came Wednesday to director-choreographer Bob Fosse, just as the opening-night curtain was going up on "Sweet Charity," the reminder was all the more acute. Hours before, Fosse had been fine-tuning the show, whipping it into a state of adrenalin-fueled alertness for the 1,800 or so spectators who would soon be settling into seats at the National Theatre.

Performers have a hoary axiom to deal with the tragedies that intrude on the well-ordered universe of the theater: "The show must go on." Indeed, it must. It is all the show can do. Unlike the cinema, which stores its treasures in vaults or transfers them to videocassettes, the theater's past cannot be preserved. Its dreams and intuitions and visions take human shape, breathe in the air of the moment -- and vanish.

The playhouse is the home of the evolving present. The energy generated on a stage encounters the energy emitted by an audience, and out of the collision, theater is born. Or that night's version of it, which may not necessarily be the next night's. We sense that deep down -- it is, in fact, precisely what fuels our exhilaration. It is also why the death of a man of the theater catches us up so short.

In Fosse's case, a partial legacy exists on film -- you can see him dancing in some of Hollywood's 1950s musicals; he made what may be the best cinematic version of a Broadway musical ever, "Cabaret"; he poured many of his personal obsessions into the gritty "Star 80" and in "All That Jazz," dared to make portray himself as callous and furiously driven. His greatest achievements, however, were forged in the theater, and in the theater, you are writing on water.

We like to think that revivals will be able to pluck a fond memory from the past and make it shine like new. But a "West Side Story," say, can no longer offer us the original choreography by Jerome Robbins -- only the original choreography "recreated" by someone else. At best, Gower Champion's direction of "Hello, Dolly!" is reproduced by an industrious assistant, who does his level best to get it right. The "Cabaret" currently at the Kennedy Center features sets based on the late Boris Aronson's designs. This sort of thing happens all the time and the results are never completely satisfying.

It is akin to watching a well-meaning impersonator lip-syncing a record. The attitudes have been meticulously studied, the mannerisms faithfully duplicated. Even the costume is button-perfect. And for a while, you actually get caught up in the illusion. But the illusion develops telltale cracks and the moment of rude awakening inevitably arrives. What can never be recreated, you see, are those initial animating sparks that bring a play, a dance, a performance into being.

"Sweet Charity" was a big show for Fosse -- it ran 608 performances in 1966 and thrust him into the special ranks of director-choreographer. But some two decades later, when it was decided to bring the musical back to Broadway, even he couldn't recall exactly what he'd done the first time around and relied on his ex-wife and the show's original star, Gwen Verdon, to help fill in the blanks. Nonetheless, it was Fosse, not a devoted acolyte, replicating Fosse and the spirit of his work was as authentically raffish as ever. The saga of a gallant dance hall hostess searching for a tidbit of happiness in a heartless world still set his imagination churning.

Fosse wasn't an intellectual and he never encouraged you to think great thoughts. But he celebrated show business in all its spangled, overpowdered, step-right-up-folks bravado. He worked with top-flight professionals and drilled them unmercilessly. But his heart lay with the carnival barker, the taxi dancer, the sweaty stand-up comic and the second-rate vaudevillian, whose moxie outstripped their sorry talents. He was the great slummer, who found a higher calling in the lower forms of entertainment.

From the late 1950s on, I managed to see most of the musicals Fosse worked on (and a lot more I wished he had). Some were better than others, but they all bore his unmistakable imprint -- a choreographic fascination with the tic and the twitch, a love of neon-lit razzle-dazzle, a side-long grin and, of course, a profusion of derby hats and canes.

One of my earliest theatrical memories is of Carol Haney and her two male sidekicks, skidding on their knees and hissing out the "s" in "Steam Heat" in "The Pajama Game." I remember Verdon leading the chorus through the blazingly patriotic strutting of the "Uncle Sam Rag" in "Redhead." And I remember being knocked out by the ending of "Pippin," for which Fosse banished all the scenery, killed the stage lights and stripped the very shirt off the back of the hero, who had wanted to do such great things with his life. (Instead, he was left on the bare stage to utter a pathetic little "Tah-dah!")

I remember how garishly amusing and giddily evil matters got in "Chicago" just as I remember thinking who else but Fosse would have the prisoners in a chain gang, manacled at the feet, indulge in a collective soft shoe -- one of the better numbers in last year's "Big Deal," his final Broadway show.

But I would be fooling myself -- and you -- if I pretended to recall more than that. A detail, a climate, an aura, yes. The whole picture? No. As an adjective, I suspect "Fosse-esque" will stand for a long time in my mind to designate something crisply seedy, disreputably cocksure or stylishly disjointed. Otherwise, the specifics will continue to blur.

In the theater, the living thrill is quickly transformed into the recollected thrill, which eventually dims, too. Then the history books take over. The only consolation to be derived from the death of a showman like Fosse is the knowledge that he provided us with some "good times, great times" -- as the taxi dancers in "Sweet Charity" call out to the prospective big spenders in the audience.

Of course, the show will go on. It is the theater's destiny to renew itself or perish. The magic will happen again in some other place under other, as of now, unforeseen circumstances. I'd like to think we'll be wise enough to drink it in, enjoy it fully, cherish its rareness. The most the theater can do is create a moment for the moment. When the moment's over, it's irretrievable.

And then we all go up the aisle and out into the darkness.