When Robert Downey Jr. saunters into the room, you first check out his chosen appearance for the day: Jaunty straw fedora garnished with a colourful band, dark glasses, blue T-shirt, black suit jacket, jeans, neatly trimmed mustache and beard, playful expression. And then you take in the mysterious black box which sits on the table in front of him for the next half-hour. So the obvious question arises - what's with the box?

"This recent maelstrom of success has put me in an interesting political position," Downey replies, voice solemn but eyes dancing. 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."

Downey could be dredging up lines from his huge summer success, Iron Man, or even giving us a glimpse of the script for Iron Man 2, which is undoubtedly going to happen. Or maybe Downey is just having fun with reporters. He's certainly wired today - here ostensibly to talk about the upcoming Tropic Thunder and a performance which is already causing controversy, but unable to resist various Downeyesque riffs about the state of his career and the world. In the course of his freewheeling session, we not only learn a lot about Tropic Thunder, but also get the word on the next Iron Man adventure, his coming assignment as Sherlock Holmes for director Guy Ritchie and his determination do a musical. In brief, the guy whose career was tanking only a few years ago because of chronic substance abuse troubles is very much on a roll. But he can still be counted on to supply his own sardonic spin to the fact that he now enjoys superstar status.

"Sometimes that's like - can you enjoy the good times right now? The same as whether you can enjoy the bad times?" He's now in a stream-of-consciousness mode. "I think you can have a good day regardless of what's happening... I think there's more pressure to have a good day when things are good because you are supposed to feel a certain way. If you don't feel that way, does that mean there's something wrong with you? No, it means that you have the whole thing wrong, which is that, good day or bad day, the way that you feel is the way that you feel and it usually doesn't match up."

Meanwhile, he feels pretty good about Tropic Thunder despite the fact that he is one of the most politically incorrect components of a politically incorrect film. Ben Stiller's comedy sends up both the cult of war movies and the more ridiculous pretensions of actors, by chronicling the disaster which ensues which a group of preening, self-absorbed Hollywood stars set out to make a Vietnamese war epic.

Downey plays a narcissistic five-time Oscar winner who decides on this occasion that he will best serve his art by darkening his complexion and rumbling his way through the role of a black soldier named Lincoln Osiris - in fact the obsessive method actor he portrays is so dedicated to his craft that he refuses to drop out of character for the entire shoot.

And what did Downey think when he first encountered this guy in Stiller's script?

"I thought like that thing when someone says, 'Hey, you want to come to dinner?' You say, 'I'd love to.' They say: 'Great and would you like us to soak you in lamp oil and light you on fire right after dessert?"'

Downey did read the script through and thought it was funny. "But the idea of actually playing the actor who's playing a black guy is more of a fun read than it is a good idea for a thing to do in your career. Maybe it goes well and you can open up the Los Angeles Times and read that Robert Downey Jr. is flat- out hilarious, like I did this morning - or you're vilified for having made a decision to do something that people thought was offensive."

Downey being Downey, he decided to do it. Besides he enjoys all the hazards involved in picking his way through a minefield. Take the Sherlock Holmes movie which starts shooting October 6. If you can believe Downey, his mind's a total blank when it comes to figuring out how to portray the legendary sleuth.

"With Sherlock Holmes, I'm like: 'Okay they're going to pay me to do this movie and I don't have a f---ing idea in my head."' He's even having trouble figuring out his English accent. "I go: 'Wait! maybe it should be like this.' Then: 'No! You don't want to do that!' Damn it! I hope I figure this out before we start shooting... Now, I'm in fear and anxiety."

What he does know is that Sherlock Holmes will be a brand new script and not based on any of the Sir Arthur Conan Doyle stories.

"It's interesting and very bold," Downey says, adding that the film will delve further into the relationship between Holmes and his faithful friend, Dr. Watson, a role that has yet to be cast.

Downey will be doing the Holmes film and the Iron Man sequel back to back. He doesn't have much to say about the latter except to firmly quash rumours that The Hulk will show up as a character. However, he does concede that doing two big movies one after the other will be challenging.

"Sometimes it's hard to stay grounded at the best of times," he notes. This time he suspects he'll find himself in what he calls a Wizard of Oz situation where "your house is spinning." At such times, in order to maintain his equilibrium, he reminds himself of the things that really matter. Am I cool right now? Am I being honest right now? Am I taking care of myself right now? Have I offended anyone? Do I need to make an apology to anyone? And have I eaten before noon?"

Furthermore, as he looks ahead, he sees a musical as a definite part of his future - "but please don't hold me to that!"

As for the contents of that black box, Downey remains tight-lipped as he leaves.

"I'm not going to tell you what's in the box! This is literally the only mystery that I have left!"