They shot it all on the back lot at Warners. They used dressed-up extras for the Arabs. The airport was a miniature and part of the town was made of cardboard. They never knew from one day to the next how the whole thing was going to turn out -- whether Ilsa would go off with Rick or with her husband, Victor Laszlo. Just play it, you know, in between, the director had told Ingrid Bergman.

She went with Laszlo, as any film nitwit knows, partly so that Bogie, standing out on that pea-soup runway in his snap-brim fedora and turned-up raincoat and hard-boiled cover, could grab the most beautiful woman on earth by the shoulders, lean six inches from her nose, and tell her she was getting on that plane. . . .That if it left the ground and she wasn't on it, she'd regret it, maybe not today, maybe not tomorrow, but soon, and for the rest of her life.

And ours, too. Here's looking at you, "Casablanca."

So whom can explain it? In Mexico once, in an expatriate artists' colony, they had a riot over the film. (The auditorium held 250 people, and thousands wanted in.) At Harvard, where they show it at least twice a year, audiences regularly shout out the lines along with the characters. (Claude Rains' "Round up the usual suspects" always brings the house down.) They do that at Yale, too, and in San Francico and Washington and probably in Kokomo, Ind. There are people out there who have seen it more times than they have seen their grandmothers. One of them is probably Woody Allen, who made a two-hour cinematic valentine to it called "Play It Again, Sam."

At the Smithsoian two nights ago, the audience merely hissed at the hated Major Strasser. And applauded wildly when Paul Henreid led the "Marseillaise" in the Cafe Americain. If you strained, you could hear a stifled sob or two.

Play it once, Sam, for old time's sake.

I don't know what you mean, Miss Ilsa.

Play it, Sam. Play "As Time Goes By."

Hokey isn't even the point. Neither is shameless romantic melodrama. Everybody knows "Casablanca" is all of those things, not to mention illogical in places. The point is graspable values, as a film student once put it: Love and honor and courage and all the old verities that it's somehow not cool to talk about anymore. Things you learn with your heart, not your glands.

The film is about how we thought we were, someone else has written. In a world seemingly incapable of hero worship, the move works like a dream in Electraglide. Two years ago, the American Film Institute named it the second-best American movie of all time, behind "Gone With the Wind." Somehow, "best" seems irrelevant. Just call it the most popular.

I stick my neck out for nobody.

"You know, I almost think audiences go to it therapeutically, to find out what is worth celebrating," says the man who wrote the screenplay 37 years ago, Howard Koch. "Sometimes I get a superstitious feeling about it, a mystical feeling. Maybe it needed to be written all along. Maybe it had a life of its own, beyond me or the director of the people who played in it."

Howard Koch is 77 now, a man who has aged with grace, no matter the years in the McCarthy era when he was blacklisted with Dalton Trumbo and others, when he couldn't find work and had to move to England, when he went from $3,000 a week to nothing a week.

"When I introduce it now, I feel I'm among friends," he says.

The other evening, Koch introduced the bit of immortality he was privileged to script (along with Julius and Philip Epstein, who had written early drafts). The audience welcomed him like a lost brother. Koch said that originally Jack Warner wanted George Raft and Hedy Lamarr for the romantic leads, then Dennis Morgan and Ann Sheridan. Dennis Morgan??? Saying lines like, "Look, I'm no good at being noble, but it doesn't take much to see that the problems of three little people don't amount to a hill of beans in this crazy world?"

The audience cackled at that proposition. They hooted when Koch told them that, for a time, Ronald Reagan was considered for the part of the antifascist, super-idealistic Laszlo.

Maybe the real genius of "Casablanca" is its casting. It's hard to imagine anyone but Bogart playing Rick, Bergman playing Ilsa. But the other roles were inspired, too -- from Sidney Greenstreet as the choice for the oily Ferrari to Peter Lorre for the even more oily Ugarte to the then-unknown Dooley Wilson for the loyal-to-the-last Sam. When the film premiered in 1942, Bosley Crowther of The New York Times pronounced Wilson a "Negro find."

"I just feel it's one of those pictures that should never be made again," says Koch. One winces at the thought of a "Casablanca II," starring Burt Reynolds and Cybill Shepherd.

Yesterday, Howard Koch sat in his hotel room, reminiscing. He down-played his contributions to the film. In a way, he seems as gentle and charming as the movie he wrote. On a bureau there was a copy of Koch's recently published memoirs, "As Time Goes By." His publisher is sending him around to chat it up these days. You got the idea that the man who wrote "Sergeant York" and a dozen or so other films would just as soon be in Woodstock, N.Y., where he has lived since he came back from England.

Koch also scripted the Oct. 30, 1938, radio broadcast of Orson Welles' "War of the Worlds" (also titled "Invasion From Mars"), which had scads of people terrified that an interplanetary war had just been touched off in New Jersey.

Koch says he was blacklisted for working for democratic labor unions and for worrying about things like pollution. Unlike Trumbo, he never joined the Communist party, he says. When he got blacklisted, word was passed to him that if he went to see a certain lawyer and was willing to cough up $7,500 and say that he regretted any liberal-cause involvement, he could go back to work.

"But what could they offer me that was worth that?"

That sounds like a little something Richard Blane, that old gunrunner-turned - Casablanca - salon -keeper - and - tough-talker, would say.

What brought you to Casablanca, Rick?

My health. I came for the waters.

What waters?We're in the desert.

I was misinformed.