Behind every great detective there is a trusty sidekick. For Sherlock Holmes, it is the courageous yet grounded medic John Watson who strives to keep the mercurial sleuth in check and out of trouble.

Robert Downey Jr, the actor who plays Holmes in Guy Ritchie’s screen versions, says that he too relies on the support of another, in this case his wife Susan.

They met on the set of Gothika in 2003, which Susan was producing, as Robert was rebuilding his career after his struggle with drug addiction had seen him imprisoned, in rehab and fired from jobs.

So united are the pair, who are currently expecting their first child in February, a sibling for Downey’s 18-year-old son, Indio, from his marriage to actress Deborah Falconer, they called the production company they formed last year Team Downey.

But is this romantic and professional partnership one of equals?

Not according to Downey Jr.

“Well, first of all, I’m definitely the boss at home,” says the actor, after a mischievous glance at Susan who is sitting within earshot. 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. Where she belongs.”

He keeps a straight face for all of about two seconds before it melts into a wide grin.

Dressed in a dapper dark suit, shirt and tie, the 46-year-old boasts a devilish streak. It’s clear he’s absolutely besotted with his wife of six years, who – technically speaking – was his boss, as one of the producers of Sherlock Holmes: A Game Of Shadows.

“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 who’s 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.

"It puts responsibility on making the relationship healthy, which is good.”

A Game of Shadows is a sequel to the 2009 box office hit, also directed by Ritchie, which raked in $524 million globally.

Downey Jr’s funny and eccentric portrayal of Holmes 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 and preternatural powers of perception.

“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’,” he says.

In the new film Holmes finds himself challenged by his nemesis, Professor James Moriarty (Jared Harris).

Not only is he Holmes’s intellectual equal, but his capacity for evil and complete lack of conscience 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 Jr. “He’s focused on him to the exclusion of everything else, including his own sanity.”

Dr Watson (Jude Law) is on hand to help as the investigations take them out of London to France, Germany and Switzerland.

The winning connection between Holmes and Watson is reflected in the close friendship the two actors have forged off-screen.

“I feel about Jude the way Sherlock feels about John; I love the guy like a brother. I couldn’t ask for a better partner.”

The movie marks the addition of Noomi Rapace, who came to worldwide recognition in the Swedish film, The Girl With The Dragon Tattoo, makes her English-speaking debut as Sim, a Gypsy fortune teller.

“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 feel like we really broke water, so to speak. To have our duo become a trio was new and exciting,” says Downey Jr.

Born in New York to actor parents, he made his professional debut at the age of five in a comedy called Pound.

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.

In 2001, however, he managed to get his life back on track. With the help of Susan, whom he credits for helping him mellow, he has stayed on it.

In the last decade, he’s made 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 2009, he received a second Oscar nomination (the first was for Chaplin in 1993) for Tropic Thunder.

Now his focus is back in films, he says he relishes the opportunity to return to iconic 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.”

* Sherlock Holmes: A Game Of Shadows is released on Friday, December 16