Interview: Robert Downey Jr

“Well, first of all, I’m definitely the boss at home,” says the idiosyncratic actor. “And I’m the boss on the set. So that doesn’t leave a lot of room for discussion, does it? I mean, I’m so happy she’s sitting behind me right now. That’s where she belongs.”

He manages to keep a straight face for all of about two seconds before it melts into a wide grin. It’s clear the 46-year-old is absolutely besotted with his wife of six years, a film producer (who – technically speaking – was his boss) on their latest movie, Sherlock Holmes: A Game Of Shadows.

It’s a sequel to the 2009 box office smash, also directed by Guy Ritchie, which raked in $524m globally.

The follow-up looks set to do equally as well, and he and Susan have extra cause for celebration as they’re expecting their first child together in February, a sibling for Downey’s 18-year-old son, Indio, from his marriage to the actress Deborah Falconer.

“The Mrs and I, we’re buddies. We’d rather work together than not work together,” he continues.

“But when you’re married to someone whose central on a production, you have to make sure that anyone who’s part of that experience doesn’t wish that the two of you would stop working together immediately.”

His funny and eccentric portrayal of Sherlock Holmes in the 2009 reboot defied convention. Gone were the once-emblematic deerstalker hat, curved pipe and British decorum of author Arthur Conan Doyle’s iconic Victorian stories, and in their place was a streetwise, bare-knuckled brawler, whose physical prowess was equal to his superlative mind.

“The bar was pretty high in the first one, which bothered me because I thought, ‘If we don’t beat this or do something new or different, I’m going to be miserable’,” says Downey.

In A Game Of Shadows, Holmes finds himself challenged by a new criminal mastermind, Professor James Moriarty (Jared Harris). Not only is he Holmes’s intellectual equal, but his capacity for evil might just give him an advantage over the renowned detective. In the opening sequences, we witness the effect of Holmes having been consumed with Moriarty in the months that have elapsed since the last film.

“He’s clearly ‘nutting up’,” explains Downey. “He’s focused on him to the exclusion of everything else, including his own sanity.”

But his ever faithful friend and colleague Dr Watson (Jude Law) is on hand to help as the investigations take them out of London to France, Germany and finally Switzerland. The pair put in additional hours together on their days off, honing their characters and rehearsing scenes. And the result? Palpable on-screen chemistry.

As Ritchie says: “The bread and butter of this movie is always going to be the relationship between Sherlock and Watson, and you can’t really have enough of those two and their camp banter.”

Aside from Moriarty, the movie marks the addition of Sim, a Gypsy fortune teller who Downey calls “the lynchpin to unravelling the case”.

The role marks the first English speaking part for Noomi Rapace, who came to worldwide recognition in the 2009 Swedish film, The Girl with the Dragon Tattoo.

“I don’t think Guy or I would say it’s particularly easy to fit girl parts into a movie which has two strong male characters in it. But I’m really satisfied with the way it went. To have our duo become a trio was new and exciting,” says Downey.

Born in New York to actor parents, he made his acting debut at the age of five in a comedy called Pound. A member of the 80s Brat Pack, he went on to flex serious acting mettle in Less Than Zero as a rich young drug addict, but it only served to exacerbate his own addiction.

Dogged by his drug habit from a young age, he still somehow managed fine performances in the likes of Chaplin, Natural Born Killers and even Ally McBeal. But in the late 1990s, the headlines were more about the drug busts and jail terms than his career.

In 2001, however, with the help of Susan, who he credits for helping him mellow, he managed to get his life back on track.

In the last decade, he’s filmed serious dramas such as David Fincher’s Zodiac and George Clooney’s Good Night, And Good Luck and enjoyed blockbuster success with Iron Man, in which he starred as the Marvel Comics superhero. Now his focus is firmly back in films, he says he relishes the opportunity to return to characters such as Iron Man and Sherlock.

“There’s enough knowledge, understanding and research. It’s like a sick Dungeons And Dragons. There are all these different ways you can go, none of which are incorrect.”

Don’t call me ‘Shirly’

Stephen Fry stars as Holmes’s older brother Mycroft. It was Fry’s idea to refer to Sherlock as ‘Shirly’ in the film.

The exterior of the iconic 221B Baker Street was constructed at Leavesden Studios.

A high-speed digital camera dubbed ‘Holmes-O-Vision’ called the Phantom enabled the director to slow the pace of action.

Watson’s stag party was shot in London’s historic Wilton’s Music Hall in the East End.

Sherlock Holmes: A Game Of Shadows is on general release from today.