Backstage in a Madison Square Garden hospitality room, Phil Joanou was just tucking into a steak dinner when the call came in via walkie-talkie. An assistant delivered the message: The band was leaving the hotel in 15 minutes and wanted a film crew to shoot them riding over to the concert in the limo.

Joanou did not immediately stir. After four weeks on the road with U2 filming slice-of-life footage for a planned documentary feature on the Irish rock supergroup, the young director had learned to be suspicious of fleeting photo opportunities. He was also hungry and tired. The night before he and his principal star, lead singer Bono, had styed out late in Little Italy, eating and talking with record producer Jimmy Iovine until 4 a.m. Earlier in the day they had all been up in Harlem, where Bono and his mates -- The Edge, Adam Clayton and Larry Mullen Jr. -- had stopped by the Calvary Baptist Church on West 124th Street to trade songs with a local gospel choir, the New Voices of Freedom, while the cameras rolled for posterity. A limo ride from an East Side hotel to the concert venue, assuming Joanou could even get there in time to film it, seemed tame by comparison.

"You sure it was Bono himself who phoned?" he asked warily, wolfing down a piece of tenderloin. "He's the only who who'd keep them there. I'd hate to get over to the Omni and find out they'd all taken off."

"It was one of the security guys," said the assistant. "All I know, the message was; Be in the lobby in 15 minutes. The band wants film."

Joanou gave a shake to his shoulder-length hair and swallowed another mouthful in silence. "Both cameras loaded?" he finally asked, looking up.

The assistant nodded.

"Okay, then," said Joanou, pushing back from the table. "We're out of here." He eyed the remains of his meal almost wistfully, then turned back to his dinner companions. "It's always like this," he sighed. "Wait and wait and wait and wait and then -- pow! Be there in 15 minutes."

Grabbing a dinner roll, Joanou disappeared. Forty minutes more and he was back in the Garden, this time leaping nimbly from a long gray Cadillac as three band members -- Bono, Clayton and Mullen -- spilled out of the back seat and wandered off to their dressing rooms. Clayton and Mullen were gone in an instant, but Bono lingered to greet some guests, and Joanou's crew was all over him: two cameras grinding away front and back; kleig lights glaring; Bono's every twitch recorded in 16mm black-and-white. In the heat of the action ("I'm going for the World War II battle footage effect," Joanou explained later) the director became rejuvenated. Loping back to the production office, in fact, he looked even more animated than his leading man, the reigning cover boy of rock 'n' roll.

"Unbelievable stuff!" he crowed, bouncing up and down on the balls of his sneakers. "Larry and Bono cracked jokes the whole way over -- I've never seen larry that loose. Or funny. And Bono, he was fabulous. Know what he wanted to do? He wanted to pull the limo over to the curbside, find some punter -- that's what they call U2 fans -- haul him inside and take him to the show. Can you imagine that? Some kid in a U2 T-shirt is walking along and all of a sudden this limo pulls up and the door opens and Bono reaches out and pulls him inside and drives him to the stage door! I mean, talk about your mind being blown! I can't wait to get that on film."

Asked why he'd had to wait this time, Joanou smiled. "Unfortunately," he signed, as if neighborhood explained everything, "we were on Park Avenue at the time."

Under slightly different circumstances, Phil Joanou might have considered turning the lens on himself. At 25, he is roughly the age of the average U2 fan, and his passion for their music is no less intense; growing up as a teen-ager near Los Angeles, he often camped out overnight by the Ticketron window to secure decent seats to their shows. More to the point, his directorial presence on the band's current U.S. concert tour seems as spontaneous as a tumble into the back of a limousine.

Three years ago, Joanou was just another precocious undergraduate at USC film school when his senior project, a 30-minute movie titled 'The Last Chance Dance," made it into a select group of student films screened for the Academy of Motion Picture Arts and Sciences. A cassette found its way to Steven Spielberg, who, ever eager to discover and promote young industry talent, signed up Joanou to direct two segments of his "Amazing Stories" television series. In short order came an office for Joanou at Amblin, Spielberg's production headquarters on the Universal studios lot, and a shot at his first commercial feature, the just released Universal picture "Three O'Clock High."

In part because of the Spielberg connection -- call it U2 meets E.T. (sort of) -- the band and its management team invited Joanou in last May to pitch them on ideas for the proposed documentary. Money was not really an issue. Having already interviewed several "name" directors, U2 was firmly committed to financing the estimated $3 million project out of its own ample pockets. "Final cut" was ;not a negotiating point, either. By fronting the money itself, the band was determined to keep creative control over the film inhouse -- the same degree of control it has exercised over everything from interviews and album covers to the T-shirts and other merchandise on sale at its concerts.

What U2 was looking for, according to Joanou, was a true collaborator -- someone skilled in the techniques of filmmaking, sensitive to the often reclusive personalities of the musicians themselves, and open to suggestion about what shape the film should take as U2 rocked and stomped its way across America.

"I'm not a groupie with a camera," he maintains. "I have a definite idea of what I want this film to say -- musically, intellectually and emotionally. But this isn't '60 Minutes,' either. I'm not out to embarrass these guys in any way, or to pry into their personal lives where it isn't appropriate. It's their movie.

"When we first starting talking about it," Joanou adds, "their two biggest concerns were one, not making a film that would bore their audience, and two, not making just another concert film. And, face it, if they'd wanted some slick, MTV-type video, there are about 2,000 directors out there with more experience doing those than I have. But they didn't. They were after something with the same rawness and degree of intensity that U2 projects on state."

Discussions between band members and filmmaker wound up spanning four months and a couple of continents. Joanou journedyed to Dublin three times -- his first visits ever to Europe -- to hang out with the group, spending time in their homes and swapping informal opinions with them about music, film and music films. Although nobody in or around U2 had seen a print of "Three O'Clock High," all apparently felt comfortable enough with Joanou personally, and trusting enough of his still thin track record, to keep the dialogue alive.

"Phil is an extremely bright kid," offers U2 publicist Paul Wasserman. "Plus he's everybody's idea of this year's fantasy career in the film business. Look, he goes to film school, does brilliant work there, gets plucked away by Spielberg, and -- zap! -- he's a major director. I'm sure the guys could identify with Phil as a young man of the doorstep of superstardom."

The pace of the decision making stepped up markedly in early September, when Joanou flew to the south of France to engage in final negotiations. Even then, the issue was in doubt. A day after he arrived all four band members headed home to Dublin, where they voted on Joanou's candidacy in private. Joanou got the election results in London, on a Friday. By Saturday, he was on a plane to New York City. By Tuesday, he was filming.

Since then, it has been a long string of 18-hour days for Joanou, who plans to put a million feet of film in the can between now and the end of December. Some of that footage will appear in black and white, blow up to 33mm in the lab to achieve what Joanou characterizes as "a gutsy, grainy effect." Scheduled vignettes include U2 performing with that Harlem gospel choir, U2 appearing with veteran bluesmen in Chicago and the Deep South, and perhaps -- who knows? -- even U2 mauling a punter in a limo. These segments will likely be intercut with full-color concert sequences, which are to be shot at one or two venues later in the tour.?

Joanou readily concedes that the physical demands of touring and movie making simultaneously are far beyond any he endured during his first location shoot. Moreover, he says, the bans's priorities -- doing a concert tour first, making a documentary film about it second -- place him in a curious position.

"It's like shooting your actors all dy and then having them go out and perform a play in the evening," he says. "They're not on call until noon every day, but my crew has to be ready by 9:30. And even then, you never know. One day we're following them through the streets of Harlem, the next day they never come out of their hotel rooms. It's never real predictable."

One of the least predictable moments occurred during U2's concert last month in Washington. Halfway through the show at RFK Stadium, Bono slipped on a rain-slickened ramp and suffered a separated shoulder. Although Joanou wasn't filming the actual show, he jumped into an ambulance with the singer afterward and, accompanied by a D.C. police escort, followed him into the emergency room with camera running.

"Bono has been in considerable pain ever since," he says, "but I kind of think that's only given his performancees that much more intensity."

"Intensity" is a word one can't seem to avoid in conversations with Joanou. Ask him for a list of rock concert films he most admires, and he quickly mentions the "intensity" of three consensus classics: "Gimme Shelter," the Maysles brothers' film about the Rolling Stones' 1969 U.S. concert tour; Martin Scorsese's "The Last Waltz," a documentary about the Band's Winterland farewell concert in 1976; and Jonathan Demme's "Stop Making Sense," a rollicking look at the Talking Heads onstage (a fourth contender, Taylor Hackford's "Chuck Berry Hail! Hail! Rock 'n' Roll," which features Berry and some famous friends, has just gone into national release.).

Might he be tempted, Joanou is asked, to assume an on camera interviewer's role, a la Scorsese with the Band? He says no. "'The Last Waltz' is probably the best concert movie ever made," says Joanou, whose affection for Scorsese's work is enormous. "But that was a once-in-a-lifetime musical event, and it needed the framework Scorsese imposed on it. 'Gimme shelter' had the raw intensity of the '60s going for it. 'Stop Making Sense' was really an art film -- a great art film, mind you, but an art film nonetheless. You wouldn't take the same approach to Bono that you would to David Byrne, because they're two completely different kinds of performers.

"We're not lighting (U2's) shows for film," he continues, "at least until we get to the color stuff. We're not taking any cameras onstage. We won't be going onto some sound stage later and recreating concert footage. None of the band members plan to play to the cameras, and I don't want them to. What's appropriate for the Talking Heads isn't necessarily appropriate for U2. If we do this thing right, it won't look like anything you've seen before.

What it might look like is hinted to by "Three O'Clock High," which was released nationally just last week. While the film may not make its neophyte director famous-made for just $6 million on a tight 34-day shooting schedule, it is, by Joanou's own admission, "an underdog movie with no star actors or big-name director to carry it along at the box office"-it does manage to take a high-school-movie premise and give it the kind of jagged, paranoid edge that a young Scorsese might have aimed for. Certainly Joanou's visual stamp is everywhere on it. His camera almost assaults the actors, lunging and looping like a predatory bird circling in for the kill.

At one point in the plot line, Jerry Mitchell, the film's protagonist, is confronting the full horror of a day gone impossibly wrong: embezzled funds, concealed weapons and an impending fight to the death in parking lot with a bully of Frankenstein proportions. Two classmates corner Mitchell and attempt to cut a deal. Since both of them are applying to UCLA film school, they argue, why not let them follow Mitchell around with a camera and document his near-certain demise.

"Pain is temporary, Jerry," one of them says, voicing an ethic heard often in Hollywood, "but film is forever."

That character could have been Joanou himself not too long ago, dragging a Panaflex into the the gymnasium and pointing its lens at a pep rally. But it's not quite that way today. At Philadelphia's JFK Stadium last month, Joanou found himself crouched six feet from center stage, shooting Bono at waist level. Suddenly he was inspired to do a slow pirouette with the camera.

A sea of screaming U2 fans-86,000of them-appeared in his viewfinder. All of them were standing not by but on top of their seats, singing along at full decibel. A thousand Bics were blooming. The old football stadium practically lifted off its moorings. Framed between the band and the crowd, Joanou felt as if he could have gone either way-into the seats or up with the stars. It was, he will tell you, intense.