Ben is reaching for the dark side

Ben Stiller lopes into the hotel suite, his shoulders around his ears, his gaze set to freeze.

He's on his guard and I'm on mine. In the days before our interview I run into three separate people who have met him before.

Away from the cameras, I am warned, Hollywood's court jester is no jester at all. He is, they tell me, a clenched, complicated individual; an unwitting comedy superstar and possibly an unwilling one, too. Stiller allows this, up to a point.

"I totally get why people see me as the comedy guy," he sighs. "That doesn't mean I don't have different ideas of where I want to go."

At the age of 42, he has found a lucrative run of form as the ultimate anti-star of American cinema; Hollywood's outsider on the inside; the frustrated everyman or the ludicrous buffoon. And, he is not knocking this success, exactly. It just isn't quite how it was meant to be.

So what is he to do? Tropic Thunder, his latest film as a director, hardly signals a U-turn. It is a big, loud, fitfully hilarious blockbuster about a bunch of preening megastars who get lost in the jungle while shooting a Vietnam war epic.

Stiller headlines as a fading action hero named Tugg Speedman, while Robert Downey Jr co-stars, in blackface, as an Australian method actor, who may (or may not) be based on Russell Crowe. "I don't read the script," Downey Jr mutters at one stage. "The script reads me."

Stiller explains that he actually had the idea for the film 20 years ago. In the late 1980s, he had a small part (billed 12th) in Steven Spielberg's Empire of the Sun and auditioned (unsuccessfully) for a role in Oliver Stone's Platoon. This was the era of Hamburger Hill and Full Metal Jacket, when Hollywood pups were getting packed off to actor's boot camp and then insisting that it was, y'know, just like the real thing, man.

"There's something funny about actors who take themselves too seriously," Stiller says. "It's a strange job, acting in a war movie. It's about pretending to be a tough guy from a very pampered position."

At the time he envisaged Tropic Thunder as a short three-minute skit. He's come a long way since then. Oddly enough, the spectacle of a white actor in blackface caused barely a ripple when Tropic Thunder was released in the US. Instead, the controversy was sparked elsewhere.

Stiller's movie contains a film-within-a-film called Simple Jack, in which Tugg Speedman makes a blatant bid for an Oscar by going "full retard". Simple Jack is a stammering village idiot who coos winsomely over his mother's deathbed and tells the local beauty that she "m-m-makes him tingle". I found this funny; others did not. At the film's premiere, a coalition of disability advocacy groups organised a picket, insisting that "the R-word is hate speak".

"I didn't see it coming," Stiller confesses. "I mean, we had hundreds of test screenings and we handed out questionnaires afterwards. And the issue never came up until a week and a half before the film came out - and that was because someone saw the marketing material and took it out of context," he shrugs. "If you take it out of context then of course you're going to have a problem. But I stand by the movie 100 per cent. To me it's very clear that the joke is at the expense of the actors."

All right then, let's look at the actors - and one actor in particular.

Among its gallery of grotesques, Tropic Thunder makes room for a jaw-dropping cameo from none other than Tom Cruise.

He plays a bald, boorish studio boss; a monstrous alpha dog who orders underlings to punch directors in the face and celebrates each success by dancing exuberantly in his office.

It's hard not to be shocked by Cruise's physical transformation, this clownish change of gears, but Stiller insists it's not really so surprising. "Even someone really serious like Daniel Day-Lewis has a vein of humour in his work," he points out. "That's what makes it human."

Even so, the Cruise connection strikes me as significant, in part because the two actors have a long and intertwined history.

Back when he was acting for Spielberg and auditioning for Stone, Stiller took time out to shoot a brief parody of Cruise's performance in The Color of Money. More recently he played the star's fawning body-double in another comedy short, Mission: Improbable.

"Yeah, that's right," he says. "And fortunately, Tom saw those films the way they were intended. They were never mean-spirited. They were always affectionate because I've always been a fan."

Judged on face value, these spoofs are funny because they feature a nerdish schlub who dares to impersonate a Hollywood heartthrob.

But they're more subversive than that. In terms of looks, Cruise and Stiller are not wildly dissimilar. Both men stand a reported 5'7" in their stockinged feet.

They share the same gleaming white overbite, the same radioactive stare. One might even imagine a parallel universe where each man has the other's career.

I tell Stiller that he looks like Cruise, and he barks with laughter. I can't tell whether he's embarrassed or delighted. "Right," he says. "Right. I don't think so. I mean, I understand there's some area where it crosses over. But he's in a very different place from me. Whatever connection we have is based on my admiration for his career."

By now he's practically spluttering. Cruise, he enthuses, is so smart, so committed. "When he was coming on the scene he was sort of like a matinee idol, this sex symbol. But he chose to work with really interesting directors, and that's why his career has had that longevity. It's a hard business. You have to make choices. And I really admire these people who made their choices early on, who had that instinctual sense of where they wanted their careers to go."

Was that what he did himself? "No!" says Stiller. "That's what I'm talking about. No! I've had a trial-and-error career."

He was born to a showbiz family, the son of the stand-up duo Anne Meara and Jerry Stiller. You could say that comedy is in his blood except that he has always seen himself, first and foremost, as a director.

In the early days he bobbed between jobs and genres. He made a sketch show for MTV, directed Jim Carrey in The Cable Guy and played dramatic roles in the likes of Next of Kin, Permanent Midnight and Your Friends and Neighbours.

"It was a weird development process, and I was just trying to figure it out. Then all of a sudden you do a couple of comedies, and that's that. It defines who you are and how people see you. But I have to accept it," he shrugs. "Even if that wasn't my plan - even if I wish people didn't see me like that." He pauses to take a gulp of coffee; another hit to keep him buzzing.

Maybe it's the tension that keeps him compelling. In later years, Stiller Sr enjoyed a grand Indian summer playing George Costanza's splenetic, hair-trigger dad in 28 episodes of Seinfeld.

On-screen (and off it too), his son seems harried by the same barely contained irritation.

Ben Stiller gives the impression of being a reluctant butt of the joke. His clown suit pinches and the jester bells ring infernally in his ears.

Perversely, this makes him seem all the more funny.

"Darkness," Stiller muses. "That's the direction I want to go in right now - both as a director and as an actor. I mean, I'll always go back to comedy because it's part of what I like to do. But there's this whole other side of me that I'm drawn to and that I just haven't been able to do. I have to get the balance right," he says. "And it's hard. It's really hard."