NASTY.

Satanic. Creepy... not words usually associated with Benedict Cumberbatch, but these are the adjectives the actor's chosen to describe his latest character, the monstrous dragon Smaug in The Hobbit: The Desolation Of Smaug.

"Smaug is the ultimate symbol of the corruption of power," says the 37-year-old star. "He's a sleepy serpent on top of his pile of gold. It brings him nothing but a damp, dank retirement, no joy or humour. He's vainglorious and proud of his own power and wealth, but it has essentially ruined him."

In the second part of the blockbuster trilogy, which is set in Middle-earth 60 years before the time of The Lord Of The Rings, a younger Bilbo Baggins, played by Martin Freeman (he was older and played by Ian Holm in the 2001 film), journeys with the wizard Gandalf and 13 Dwarves on an epic quest to reclaim the Lonely Mountain and the lost Dwarf Kingdom of Erebor - and slay Smaug while they are at it.

Of course, Cumberbatch and Freeman are old pals, having starred together as Sherlock Holmes and Dr Watson in BBC One's Sherlock since 2010.

But this time, while Freeman, albeit in rather distinct costume, is recognisable on screen, Smaug is CGI-animated, so Cumberbatch did his acting in a recording studio.

Getting the dragon's voice just right was important.

"I was trying to articulate certain things which are impossible, because I'm a mammal trying to be a reptile," he recalls, laughing. "It had to sound like he had the growl of an atomic explosion."

There were other, rather unhygienic factors at play in the making of Smaug's voice, too.

"It had to build that bridge between animal and human, something guttural, deep and rasped, kind of dry as well because of all the fire breathing," says the actor, who worked with a dialect coach to perfect his performance.

"I don't think Smaug's cleaned his teeth in a while," he adds. "He's got goat and dwarf breath."

As unpleasant at that sounds, it's unlikely to dent Cumberbatch's present appeal. Since his breakthrough role as the eponymous detective, the London-raised actor can barely put a foot wrong.

He's had leading roles in The Fifth Estate, as WikiLeaks founder Julian Assange, Star Trek: Into The Darkness as Khan, plus War Horse, and then there's the upcoming drama August: Osage County, with Meryl Streep.

Despite all of his credentials, when it came to filming The Hobbit, he was studious, especially when he considered the performances of other actors before him.

"I'd never worked with motion capture before," he says. "I know I'm not Andy Serkis doing amazing, pioneering and masterful work as Gollum. I'm going to be mucking around as a serpent, and wriggling around on the floor trying to articulate things I can't do."

CGI may have been new to him, but the tales of Bilbo and co were familiar.

As the only child of two actors, a young Cumberbatch was treated to tantalising bedtime stories, and one in particular stuck with him - The Hobbit.

"I was about six or seven at the time, but my father sowed the seed," the actor, who later attended Harrow School, notes.

"He's a great voice artist; he's a great actor, so he installed a love for literature in me.

"My father reading me The Hobbit was the first time I'd ever heard the written word, and I had an explosion of imaginary landscapes in my head and thought, 'Wow, that can come off the page of black and white, that's insane'," he continues, recalling his father, Timothy Carlton's influence."

More than three decades on, he's still grateful for the part his father played in steering his career.

"I owe a lot to my dad, so in a way, this feels a bit like an homage to him," Cumberbatch adds, smiling. "Or at least a doffing of my cap with some welly and gusto."