Jude Law is back to his usual handsome self after his stint as a rugged submariner for new film Black Sea, but his memories of the shoot are still vivid - including his time as a "sun dodger" on a Royal Navy sub while preparing for the role.

"Yeah, I disappeared under the waves for four or so days," the actor says with a smile, his accent returned to its London roots and his tousled hair fully grown back after following his transformation into shaven-headed, Aberdeen-born skipper Robinson.

"I felt like I fitted in quite well, really. There was very good company, incredibly weird hours - it's sort of six hours sleep, six hours awake, six hours sleep, so you do lose a sense of time.

"It is its own world, and what you realise is, ultimately, you're living in a machine - there's no sort of separation between the mechanics of it and the comfort of it. The comfort is fitted in around the mechanics."

As the film begins, we see ex-Navy man Robinson laid off from his job at a salvage company, his relationship with his wife and son already destroyed after nearly 30 years of voyages.

He learns that there may be World War Two gold lying in the Georgian depths of the Black Sea in a sunken German U-boat, and with nothing to lose and everything to gain, he decides to go and find it.

Robinson pulls together a crew of mainly British and Russian out of work sailors and acquires a rusty, just about seaworthy old submarine. But rows - and violence - soon break out underwater over how the gold should be divvied up if they ever reach it.

Meanwhile, the normally reliable Robinson begins to make reckless decisions as his judgement becomes clouded by the prospect of treasure.

This submariner is "hard-working, prideful, stubborn - and flawed, like all the great characters".

"Robinson has moments of great heroism," adds Law. "He's able to harness the powers of his crew and move them in the overall direction he wants to go. But you also realise that his obsession with succeeding in this mission, and with what this mission would bring him, could override the concern he has for the men on board.

"It's a thriller, an adventure," the star continues, "but also somehow socially poignant in its examination of skilled but desperate men, left to desperate measures in order to win back a little bit of reward for their life."

Spending large chunks of time working away from loved ones is something Law can relate to as an actor and father (he has two sons and a daughter with ex-wife Sadie Frost, a daughter with US model Samantha Burke, and a baby on the way with his ex-girlfriend, singer Catherine Harding).

"There's the dilemma, isn't there, in any working adult, where you're working to provide - and yet by working, you're away from the very thing you're providing for, and that can sometimes separate you or even alienate you from it," he says.

"That was a story I heard quite frequently from the men I got to meet [before filming Black Sea]. They found it hard getting back into the swing of things when they returned home. That weird separation is at the heart of Robinson, really. He desperately wants his family back, but his mission keeps taking him away from them."

Law turns 42 later this month and in recent years has spoken of his pleasure at taking on more diverse roles now he's not "that young sort of pretty thing any more" - including 2013 crime caper Dom Hemingway, for which he gained 20lbs to play the larger-than-life titular character.

The weathered man who appears in Black Sea is far removed from Law's matinee idol good looks in The Talented Mr. Ripley (the 1999 film for which he earned his first Oscar nomination), or the suave ladies' man in 2004's Alfie remake.

In fact, as Black Sea director Kevin Macdonald reveals, "many people on the set didn't recognise him until they saw those beautiful blue eyes".

To add to the film's sense of realism and claustrophobia, Macdonald decided to shoot some scenes on board a real submarine built in the Sixties, the Black Widow, moored in a river in Rochester, Kent. Other sets were built at Pinewood Studios in Buckinghamshire.

"Kevin made a decision to use the proximity - so rather than cutting chunks out of the boat we were filming on, in order to get further back and see the bigger picture, he was in with us," Law notes.

"In order to not kill each other, we had to retain a great sense of humour and also camaraderie. There was a strong sense of us being this crew and going on this adventure."

The Russian actors let their hair down with some vodka after filming.

"There were a couple of nights when the Russians celebrated, as was their wont, and they were very good at toasting. There was a little bit of that. I mean, we were submariners; as with anyone who goes to sea, there always has to be a little bit of celebration!"

Law's other co-stars include Shameless actor David Threlfall, US star Scoot McNairy, and Northern Irish actor Michael Smiley, whose late father was a submariner as a young man.

"Off-camera, we were a happy 12-man crew," says Law. "On-camera, was a different story..."

EXTRA TIME - FILMS SET AT SEA

:: Das Boot (1981) - This critically-acclaimed epic German war film, an adaptation of the novel of the same name (which translates as The Boat) tells the story of a fictional World War Two U-boat and its crew.

:: Titanic (1997) - Leonardo DiCaprio and Kate Winslet play star-crossed lovers Jack and Rose, who find love on board the ill-fated ship in James Cameron's box office smash, which saw Winslet receive an Oscar nomination.

:: Pirates Of The Caribbean: The Curse Of The Black Pearl (2003) - When the Black Pearl ship is stolen, Captain Jack Sparrow (Johnny Depp) embarks on an adventure to rescue it, as well as the governor's kidnapped daughter Elizabeth (Keira Knightley).

:: Life Of Pi (2012) - When Pi Patel (Suraj Sharma) gets stranded at sea after a storm, he must survive on a life boat with his only companion, a huge Bengal tiger named Richard Parker. The epic film won Ang Lee an Oscar for best director.

:: Captain Phillips (2013) - Tom Hanks leads the cast in this Oscar-winning drama based on the true story of Captain Richard Phillips, whose ship was hijacked by Somali pirates in 2009.

:: Black Sea is released on Friday, December 5