Downey Jr (top) hams it up in Tropic Thunder as Lazarus playing an African-American soldier (right, and below right).

"What’s in it?" I query, going for the bait.

"This recent maelstrom of success has put me in an interesting political position," he deadpans.

"Actually, I’m working for the Department of Defence. If something alarming goes down, I might have to enter my coordinates and be picked up by helicopter," he says, cracking a smile.

Downey, 43, is in a lighthearted mood. And why shouldn’t he be? He’s having one of the best years of his career, with the success of Iron Man and positive buzz on his next two movies – the action comedy Tropic Thunder, and the Oscar-bait drama The Soloist.

So how’s the talented actor holding up under all this pressure and attention? "Sometimes it’s hard to stay grounded in the best of times," he concedes. "Because that’s when things really start to begin spinning."

Having lived through various ups and downs, including stints in rehab and run-ins with the law, Downey is managing by keeping himself in check.

"I think there’s pressure to have a good day when things are good because you’re supposed to feel a certain way, and if you don’t feel that way, that means there’s something wrong with you," says the Oscar-winner.

Having gone through rehab, he’s aware that some days, the way he feels doesn’t match up with the way he is expected to feel.

And he’s okay with it.

Besides, he’s simply too busy making and promoting movies to really take much notice of the hoopla around him.

In Tropic Thunder, he plays Kirk Lazarus, an acclaimed Australian actor known for immersing himself in his roles. When playing the role of an African-American soldier in a Rambo-like action film, Lazarus undergoes a skin-darkening procedure to play his stereotypical character.

Of course, Tropic Thunder is a send-up of Hollywood action movies, with their overblown budgets, unprepared directors and prima-donna actors. It’s the brainchild of Ben Stiller, who plays a fading action star trying to revive his sagging career.

Downey admits he was a bit wary about taking on the part, fearing some audiences might find it distasteful for a white actor to play a black character, even though the spoof is clearly directed at Hollywood and not the black community.

Ultimately, Downey put his trust in writer-director Stiller, whose previous directing efforts include The Cable Guy and Zoolander.

Stiller says Downey was "the biggest bargain genius actor on the planet" at the time he was cast. (He’s no longer a bargain, with the success of Iron Man.)

As he does with many of his roles, Downey wrestled with how to play Lazarus. He focused on the voice first. Then he worked closely with make-up artist John Blake.

"I just kept following those little things," he recalls. "To me, it was much more of an attitude thing and who you’re playing off of."

Besides Stiller, the comedy co-stars Jack Black, British funnyman Steve Coogan and Jay Baruchel. Nick Nolte plays a military technical consultant with a secret. And Tom Cruise has a cameo as a growling producer fed up with the troubled production.

On the eve of Tropic Thunder’s release, Downey is anxious about how audiences will respond to his character, though he is bolstered by some positive early reviews.

"Maybe it goes well, and maybe you open up the L.A. Times and it says Robert Downey Jr is flat-out hilarious," he says. "Or you’re vilified for having made a decision to do something that people thought was offensive."

At this point, it’s completely out of his hands. Besides, he has his next project to worry about. He’s about to get started on Sherlock Holmes, in which he will play the tenacious British detective. And he’s feeling a bit anxious about playing the beloved character.

"I’m like, okay, they’re going to pay me to do this movie and I don’t have a *** idea in my head," he says, laughing. "I hope I can figure this out before shooting."

He is slated to begin production on Sherlock Holmes this month in London and New York.

"It’s not an origins story," he reveals. "It’s a brand new book, interesting and bold."

He is also excited about reprising his unconventional superhero in a sequel to Iron Man.

What’s next for industrialist Tony Stark and his alter ego is under wraps and under construction by filmmaker Jon Favreau.

"We’re hoping to take all this good will and success and do something even more kind of risky and develop the character," he says cryptically. "It’s going to be good."

Sworn to secrecy, he won’t disclose whether other characters from the Marvel universe will appear in the next Iron Man (as he did as Tony Stark in The Incredible Hulk). "I can neither confirm or deny," he says.

As he gets up to leave, I try once more to find out what’s in the box.

"I’m not going to tell you," he teases. "This is literally the only mystery I have left." – VNU