"MISSING": THE climactic moment. Jack Lemmon, playing a middle-aged American businessman, has just learned, after a desperate search, the tragic fate of his son in the Chilean coup. Woodenly; he leaves the office and starts down some stairs. His face is blank. At the landing he goes the wrong way, hesitates, turns, sets himself right. His legs aren't working very well.

"We did that three times," the actor says. "I went down the right side once and then the left side . . . I don't remember which one we used finally."

On the set that day, people were crying. A few million moviegoers will carry that scene in their heads as long as they live. For Lemmon it was work--planning which way to turn, what he would do with his hands, how much would show on his face.

Jack Lemmon is a pro.

"You need more control to do less in film. I mean, your eyeball is three feet wide up there. You have to bring yourself down to where you think you're not doing a damn thing but thinking."

Remember, this is the guy who giggled and sashayed and boggled his eyes so outrageously in drag for "Some Like It Hot," the sharp-featured finagler who leered so lustfully as Ensign Pulver in "Mister Roberts" and leered again, but moonily, in "Irma La Douce," the compulsively neat divorced man with the anxious face who fidgeted so fussily over Walter Matthau in "The Odd Couple," the brittle, quick, pointy-nosed, hard-voiced, darting-eyed wise guy of all time in "The Front Page," "The Fortune Cookie" and a raft of others.

And when you see the actor himself, you realize what that buttoned-down businessman in "Missing"--repressed and conventional and gray, feelings and body held rigid in an emotional straitjacket--must have cost him, for he himself bristles with nervous energy, throwing his arms wide, jumping up to illustrate some bit, talking fast with serious intensity.

The control required to play such a part just won't work with a grandstanding director (like the one years ago who kept tugging at the actor's pantlegs to move him around in midscene because it was easier than moving the camera).

"It was a joy to work with Costa-Gavras on 'Missing.' Thirty minutes after I met him I was ready to walk off a cliff for him. He knew when to direct and when not to, like if you see a Mack truck coming down the hill with its brakes off, you don't get out in the road and put your hand up and tell it to stop. You just get out of the way."

It is customary for actors to say they loved working with their director (if the picture is a hit). But Lemmon isn't being polite.

"When it was going right, Costa would he gives the okay sign , or when maybe I'm overacting because I didn't trust myself, he would he squints between thumb and forefinger and I'd bring it down a little. We got lucky early on. I felt I had the character, and he said, 'That's it, that's it,' and then it was a matter of degree."

And then the best part, after the scene was done:See LEMMON, G4, Col. 1 Jack Lemmon in "Missing." Spacer --Spacer -- Lemmon in "China Syndrome." Spacer --Spacer --LEMMON, From G1 The director would suggest a whole fresh approach, for the heck of it, and Lemmon would do it cold, without rehearsing. He loved it.

"I was so excited I couldn't see straight," he says. "Both versions were consistent with the character; they could both be him. Like on the staircase, which was the aftermath of a scene. Things don't happen during scenes, they happen after."

Lemmon insists that this role and his frustrated, inarticulate nuclear engineer in "The China Syndrome," coming almost back-to-back in two more or less anti-establishment pictures, were a coincidence, that he took them because the risks they offered excited him as an actor. "As a human being and also as a citizen, I'm thrilled that both of them have things to say I happen to agree with. But I would have taken both of them even if I disagreed. Like playing Richard II."

He chuckles hideously, twists his mouth and turns into a pouting Plantagenet right there on the sofa.

No matter what you talk to Jack Lemmon about, it always seems to come down to the nuts and bolts of acting. He has wanted to do nothing else since the age of 9. Forget that stuff about being discovered in the Pudding Club shows at Harvard: He'd already been doing summer stock for years.

"It's about being accepted. I started in a school play. I substituted for a kid who got sick that morning, and I didn't know the lines, and the teacher said that was all right, just walk over to the wings and she'd give me the lines.

"So they gave me the kid's outfit--a big hat that came down to here and a cape that dragged like Queen Mary or something, and I'd go to the wings and get the next line and walk all the way back out to center stage like a schmuck. And everybody laughed. I did it for 12 lines in a row and got a laugh every time. It was only because I didn't know the lines. I thought: I think I like this."

After Andover and Harvard ('47) the Boston-born Lemmon got a job playing piano and telling jokes in an East Side New York showbiz restaurant. Burlesque routines. ("No better training for an actor. That and fencing and ballet. Using your body. Control.") He did a revival of "Room Service" on Broadway. Still feeling very superior about Hollywood, he did some pictures with Judy Holliday--and got his mind changed. "I was this snotty New York theater kid, and Hollywood was only for curly-haired boys, but working with her knocked me out. She was something else."

Still, nothing much happened.

Then came one of those hiccups of fate: He did a screen test for a part in "The Long Gray Line," in which he had to age from 20 to 80. Director John Ford was looking at some rushes a few months later, and for reasons no one remembers, maybe for a gag, the assistant editor stuck the footage of his buddy Lemmon playing an 80-year-old man on the end of the reel.

"What the hell is that?" said Ford.

"Jeez, I dunno, Mr. Ford. It's some mistake."

"Well, I don't know who the hell that guy is, he's a rotten old man. But he might be a good Pulver." Which was a casting problem in Ford's next film.

So Jack Lemmon got Ensign Pulver. "Mister Roberts" put him on the map and won him a supporting-role Oscar. At that, it wouldn't have happened if the Broadway play hadn't run so long that David Wayne was turning gray in the part.

People talk about Lemmon's shift from comedy to serious roles, usually dating it from "Save the Tiger." Actually, the process had been going on gradually for years, in "The Apartment," "Days of Wine and Roses" and other bittersweet stories. His comic persona is essentially the same as his serious one: a very average man who believes with insouciant faith that he is winning even as disaster hangs over him.

"I don't have a preference. Maybe the next one will be a comedy. You just don't want to do the same thing over and over, do the, quote, Jack Lemmon part. Also, you're competing with yourself: Hot as you may be, it doesn't mean the scripts are there. You don't run across the light comedies you used to have."

For all his movie fame, his Academy Award in "Tiger," his directing success in "Kotch," Jack Lemmon never gets very far from the living theater. It's that audience, he says, the gusts of laughter that egg you on, the hushed silences ("like someone staring at the back of your head until you turn around"), the building of a great scene as each actor catches fire from the other . . . "You feel it," he whispers.

"Film is a bitch, with the stop-start, in and out of sequence. Suddenly it's magic time: You're supposed to be hysterical in two seconds. It's harder to do without building to it. The real reward, on whatever childish level it is (in falsetto, "Hello! Yes! We like you!" and he claps kiddie-style), what you're really there for is that acceptance, and you get it. But to a great extent in film, it's in the laps of your director and God."

An actor can learn the kind of control needed for movies. It took Lemmon a long time, he insists. But there's something you have to be born with, "like timing and the color of your hair," something you just know, something about selecting.

"It can be a simple sentence that makes one single point, and you build for that. You zero in on the one moment that gets that character, you go for it, that's it, man, and if you fail the whole thing is down the drain, but if you make it you hit the moon. In 'China' there was one line the guy says, 'There was a vibration.' That's all. The key to the whole thing." He repeats it with emphasis, snapping out each word, and the statement instantly becomes enormous.

He finds that Costa-Gavras has this sense. In the middle of filming nighttime street warfare, the director suddenly said, "Get me a white horse." He had the horse run wild down the firelit street.

"I don't know why," Lemmon muses. "All I know is, I look at it and it gets me, man. It's dramatic. He has that instinct. You can't learn it."

He wants to do another play in a couple of years. "Back to the well," he mutters. "That's where it is. You can breathe more fully on stage. I'd like to do 'Juno and the Paycock' again, maybe for cable. It's one of the half-dozen most important plays of the century; it changed the whole style of our theater."

On the other hand, he is acutely aware of the risks in theater. He tells of flying east to see a Broadway play, only to discover it had folded after one night. And then he tells a story about the time he was having a beer in the Ritz bar in Boston, just up the street from his home at Beacon and Exeter.

"I'm a punk kid about 18, in the Navy, 1940 or whatever year it was, and there's some people in the corner three tables away. By then I was so deep in the theater and music I was gonna be the next Gershwin, the next Barrymore, I was gonna save the music industry, save the American theater, the whole thing. Turns out it was Richard Rodgers, Oscar Hammerstein, Agnes De Mille and somebody from the Theater Guild, and they forget there's just this sailor 40 feet away, and I've got both ears like this! They got this show, it's a f------ bomb, they're screaming about changing the title, the dancers aren't gonna work. They damn near closed it. I listened to 'em about 45 minutes, it was just like this whether that show ever left Boston. The thing is called 'Away We Go.' So they decide to change the name to 'Oklahoma!' And boy, later, I thought back to that and I said, 'It's a ver-r-ry risky business we're in,' yehhh."

If he doesn't return to the stage, Lemmon might try film directing again. He had a great time running Matthau in "Kotch," except that "every time I got some totally brilliant idea I'd open my mouth and one quick gesture: a bear brushing a gnat from his eyebrow: Matthau to the life he'd say 'I gotcha.' I never even got to give him a direction."

He has his eye on the off-Broadway hit "A Coupla White Chicks Sittin' Around Talkin'," if he can adapt the script. "Wendy Wasserstein, marvelous girl, gotta love a girl with that name, a good writer, is taking a crack at it. We'd do it this summer with Jill Clayburgh and Susan Sarandon. It's the first time I've gotten close to where I can believe contemporary young women and their problems. It's funny. But there's a little screaming going on underneath."

Very Lemmon, that statement: energetic, breezy, perceptive. And excited. He can't possibly be 56. It seems only a couple of years ago that he was the musician on the lam in "Some Like It Hot," disguised as a member of an all-girl band, and Marilyn Monroe in her nightie was climbing into his Pullman bunk for a sisterly midnight champagne-and-peanut-butter snack, and pretty soon the whole bunk was full of squirming girls in nighties, and he wore this amazing expression of lust, cunning and pandemonium, a man in heaven and hell at the same time, and the audience was having a stroke, and it just couldn't get any funnier than that, but it did.