LOS ANGELES -- Lunch with Robert Downey Jr. First the handshake. He stares expressionlessly -- the calm before a storm of wackiness -- at the photographer as he shakes her hand a second too long. "Wouldn't it be funny if I just kept this up?" he deadpans as he refuses to release her hand and makes up a response for her: " 'Robert -- it's over.' "

Everyone chuckles, the PR people fuss affectionately, and he finally settles into the overstuffed corner banquette for six, sliding all around, eyes searching for mischief he can create -- or at least another quip he can deliver.

When his father, avant-garde filmmaker Robert Downey, cast his 5-year-old son as Puppy in his film "Pound," Downey Jr. may have found himself a lasting description. With his big brown eyes and playful humor, there's something of a frisky pup in the young actor.

At 25, Downey has the career and pretty-boy looks of the Brat Pack -- that often-lumped-together group of young actors who make movies about twentysomething yuppies, and in fact he's worked with some of them (Molly Ringwald in "The Pick-Up Artist," Andrew McCarthy in "Less Than Zero"; no, he was not in "St. Elmo's Fire"). But at his best, Downey distinguishes himself with a droll, ironic outlook that comes through whether he's the irrepressible, engagingly antic man on the make of "The Pick-Up Artist," the self-destructive dreamer and party boy of "Less Than Zero" -- his favorite role -- or the idealistic but grounded young lawyer of "True Believer" playing the foil to James Woods, his brilliant, burned-out idol.

In his latest film, "Air America," set in the Vietnam War era, he's Billy Covington, the crackerjack L.A. traffic copter pilot fond of buzzing truck drivers who slow down on the freeway to rubberneck at accidents. When he's tapped by the CIA to fly -- and blithely accepts with no idea he's about to be immersed in covert missions in Laos -- he's paired with an even more cavalier pilot, played by Mel Gibson.

Downey often outshines the movie he's in, and his latest is no exception. "Air America" runs out of story, but Downey is charming and funny in it and charming and funny when he talks about it. He regaled David Letterman on his show with stories about the L.A. helicopter pilot who took him up so he could see what flying was like.

"I was really lucky that I was witty that night," he says of his appearance on the Letterman show. Still, it's not hard to see how he could spend a year on "Saturday Night Live" -- though unfortunately it was 1985, one of the show's least memorable years. ("Thank you," he says, mock-wounded.)

He can sound pretty loopy when he starts waxing about philosophy and current events and "getting centered," but when it comes to his work, he's a bit more straightforward. "This time I went out and did all this research," he says of his new film. "Billy was from San Diego and he was a Navy brat. Maybe because this film was an action movie, I didn't use any of it. None of it mattered and none of it worked.

"What mattered was something that we'd come up with in rehearsal," he says, recalling the now heavily promoted scene in which the plane that he and Gibson are flying crashes into a tree and hangs upside down. Once while rehearsing, Gibson flipped a coin to determine who would fall out first. "It was a joke, and then it winds up on the screen. And it's not even about your character. It was about bringing something funny to the screen."

Before he went to Thailand last fall to shoot the film, he did some reading on Vietnam. "It's funny because I still didn't know what was going on in the movie when I was shooting it -- 'Now are the Pathet Lao on the American side? Are they working for North or South Vietnam?' Listen, we have to explain this story to the audience."

It's fair to say that his interest in Vietnam has been minimal. "For real estate," he cracks.

In fact, at the time that the United States was engaged in Southeast Asia, young Downey was making his film debut in "Pound," which also featured his actress mother, Elsie, who is divorced from his father and lives in New Haven, Conn.

Downey himself recalls little about the making of the movie, an X-rated (for the language, he surmises) take on dogs as they muse on everyday life.

"I remember wearing a blue shirt," he says. He's collaborated several times with his father, best known for "Putney Swope" and "Greaser's Palace," the latter being one of the films in which Downey Jr. appeared. Three years ago he starred in his father's "Rented Lips" -- "I play a porno actor who thinks he's Marlon Brando and always wants to talk about his motivation." And come fall he can be seen in his father's "Too Much Sun" as a Valley real estate salesman who dreams of being "an English rock star type guy. ... He longs to be this character named Phil. I wrote a song called 'Too Much Sun,' and then I did it as Phil." Downey Jr. directed a video of himself performing the song, which will be part of the movie.

The two Downeys have had very different careers. Downey Jr.'s is built on a string of commercial pictures. Downey Sr. has taken a less traditional road with quirky films that have particularly flouted middle-class values. These days he enjoys far less visibility than his son.

"It just happened," says Downey Jr. of his own career. "I would like to have done nothing but the kind of movies my father does. It didn't turn out that way."

He says he hopes one day to pursue different, more individualistic films like his father's. (He's currently co-writing a screenplay of a story set in early '60s Las Vegas in which he plans to act.) "I think I'll be able to kind of cube his efforts, hopefully," he says. "Of course we're a lot alike -- he's my dad and stuff -- but we seem to kind of represent the same thing in what we do or what we want to do: push the medium forward a little, turn some heads."

"Air America" is the 12th movie in Downey's nine-year career. He has a house in the Hollywood Hills, which he shares with television actress Sarah Jessica Parker ("Equal Justice"), drives a Porsche, doesn't eat meat or white sugar and can't believe gas was $1.74 a gallon the other day. "But it was full serve" -- he laughs -- "high octane, high performance."

He insists he's not rich. "I'm not at all. I spend it very quickly," he says, indulging his music writing by buying keyboards and recording instruments. "I probably have more money in clothes than anything else. It's all about clothes," he says with a chuckle.

Downey has been on his own now for nearly a decade and has spent most of his life bouncing between coasts. "I was raised for better or worse in a family that was kind of nomadic," he says. "I was always kind of used to being on my own."

He was a junior at Santa Monica High School when he dropped out to follow his father to New York. "My dad was going to New York to make a movie," he recalls. "My dad left home when he was 15. So he knew that it could work if I wanted it, but he wasn't imposing it." Downey moved into a Manhattan apartment that his father's employers had found, put Clark Gable posters on the wall, and started working at odd jobs like being a busboy.

He did a couple of off-Broadway plays before he got a small part in John Sayles's 1983 film "Baby, It's You." He was 18 when he won a part in the movie "Firstborn" and met Parker, who was also in the cast. She had a list of theater credits -- most notably a stint in the title role of the Broadway show "Annie." Nine days apart in age -- she's older -- they began living together in her apartment soon after meeting and have been a live-in couple ever since. "She was really cool and she had a really cool apartment -- with carpeting," he says, laughing. Though they are not married, Downey wears on his wedding-ring finger a Cartier rolling ring that she gave him.

They're going into their seventh year together. A week ago Friday, while he and Mel Gibson were chatting up "Air America" on Arsenio Hall's show, she was shmoozing with Johnny Carson on his show.

The two are also among Hollywood's most politically active celebrities. They are publicly supportive of environmental issues here, as well as abortion rights and voter registration. "We do voter registration drives outside coffee shops on Friday nights sometimes," he says. "Parker does it more than I do. I think voting's really important."

They pursue their careers separately. He hasn't been working for three months, since he finished "Too Much Sun." He does things like shop. Meanwhile Parker, back at work on "Equal Justice," has been taking tango lessons.

"You know what's so great -- how unattached you can be with someone if you're really close to them. She's doing this sexy tango dance with this actor, John Tenney, who plays her love interest," he notes. "She says, 'Robert, I'd like to go to dinner with you, but John and I have to go rehearse our sexy tango dance.' I'm like, 'All right, honey, I'll be at home. I'll see you later.' "