Paging Doctor Carter: aliens on the loose

Noah Wyle, who stars in Steven Spielberg's new sci-fi hit, Falling Skies, talks about why he left ER, the trauma of divorce and the fantastic Mr Clooney

Noah's arc: Wyle in London (David Sandison)

Two hundred and fifty five. That's how many episodes of ER Noah Wyle chalked up as Dr John Carter, over 15 bloody, rubber-glove-snapping years. It was more than anyone else at County General and, as such, it was Carter who was accorded the honour of the show's final line on 2 April 2009.

Seemingly throwaway – "Dr Greene? You coming?" yelled in the ambulance bay – it echoed the first words uttered on the show's debut in September 1994. In between, Carter was a startled constant, lurching, white coat flapping, from trauma to trauma, struggling through a karate-chopping supervisor, a stabbing, rehab, a mission to Africa and mouth-to-mouth resuscitation on a dog to emerge as the show's defining star.

So it's no wonder, when it wrapped, Wyle went to ground, or as he puts it, embarked on "an inevitable fallow period. So people could forget about John Carter". He shot a couple of tiny films and retreated to his ranch in the Santa Ynez Valley, a couple of hours up the coast from LA, to spend time with his family and assorted animals. He has a son and a daughter – Owen, nine, and Auden, six – with his ex-wife Tracy Warbin. "And I have horses and a couple of cows and chickens and ducks. And six pigs and some dogs and cats. And a goat", he says, wearily. "It's a sort of menagerie. A burden when your name is Noah, I suppose. But it's a nice life for my kids. We made a conscious effort to raise them outside of Los Angeles, of the industry. More of a rural, agrarian life."

Then, a couple of years ago, the phone rang. It was Steven Spielberg. The director and one-time ER executive was making an epic new television show and needed a hero. Having given up the rigours of serial drama for the good life, Wyle found himself on 16-hour shoots once again. Only this time, instead of grappling with gurneys in County General, he was roaming around an abandoned hospital in Toronto, firing at CGI aliens for Falling Skies. A sci-fi blockbuster set in the aftermath of an alien invasion, it premiered to 5.9 million viewers in America, making it cable television's biggest launch of 2011. A second season starts in the UK tonight.

Did Wyle have qualms about putting down roots with another long-running character? "I had an itch to get back to work", he shrugs. Why not a nice, short movie? "Not a lot of movies being made, not a lot of offers coming my way, and the ones that did weren't my cup of tea".

And so Chicago's least-likely lifesaver (at least in Dr Benton's eyes) has become Boston's least-likely alien-slayer. Today, marooned on a small chair in the middle of a vast penthouse suite at the Mayfair Hotel in London, he is dressed like an English landowning gent – desert boots, rumpled blue Oxford shirt and tweed jacket with silk square neatly tucked in the breast pocket. Still boyishly handsome at 41, he looks, unsurprisingly, just like Carter, if a little more careworn; Carter after a week of night-shifts, perhaps. Aside from an incongruous tattoo peeping out of his cuff – a large black O for Owen on his wrist – the overall impression is more historian than hero. Which is apt, because in Falling Skies he plays Tom Mason, a professor of military history who finds himself thrust, unwillingly, into real action when Boston is overrun by extra-terrestrials.

Mason is a classic Wyle hero: hapless, reluctant, brimming with decency if not derring-do. If the actor had it his way, he would be rather more flawed, but this is a Spielberg show.

"We had a fight about it. I was chomping at the bit to not play the character as such a paragon of moral virtue", says Wyle. "Michael [Wright, president of TNT] was adamant about not wanting leading men on his network to be ironic. He likes his heroes heroic."

Having been nominated for five Emmys and three Golden Globes for Best Supporting Actor while on ER, the show is Wyle's chance to play a leading man at last. "I kind of inherited ER over the years as other characters left", he says. "I moved to the centre but it was never really my show, never one that I felt I was carrying."

He took the new role partly to impress his son – "Owen gets all sorts of street cred from his dad being in a show killing aliens" – and partly because he wanted to play a father. In the show, Tom is left to bring up two sons when his wife is killed and his third son captured by the invaders. "I was curious to explore the notion of really grievous loss – when everything that you've defined yourself by is taken away from you. How do you move forward through that? What are the things you cling on to to make life worth living again?"

If it sounds like therapy-speak, that's probably because Wyle has been working out his own problems on set. In 2009 he split from Warbin, a make-up artist he met on The Myth of Fingerprints, after almost a decade of marriage. Is playing Tom cathartic? "Certainly getting divorced was rough. It all circles around that theme: you thought you were one kind of person and you promised yourself you'd never do this or that and then you went ahead and did it. What does that mean? Who are you? What are you really capable of? All that kind of stuff…"

Wyle was born in Hollywood. His parents divorced when he was young and his mother, a nurse, remarried James C Katz, a film restorationist. After attending California's oldest boarding school, the Thacher, he studied theatre at Northwestern University. He was waiting tables at Hollywood's Bel Age Hotel when he landed his big break, playing a slow-witted marine in Rob Reiner's A Few Good Men.

Aaron Sorkin, who wrote that film, recently used Wyle as an example in a graduation speech at his alma mater. They had, apparently, wanted someone else for the part but he dropped it when he won a bigger part in another film. That film never got made, the actor sank without trace and the bit part went to their second choice. "An actor called Noah Wyle", said Sorkin. "Who hasn't stopped working since".

"Really? He said that? Really?" Wyle looks thrilled. "Well it's nice to know that Mr Sorkin is still thinking about me…" This, incidentally, is a charming habit of Wyle's: Mr Spielberg this, Mr Reiner that. How polite. "I hope that that's the reputation I've established. You hire me, you don't have to worry about me: I'll be there on time, I'll be prepared. I'm personable, professional and punctual."

He was 23 years old when he landed the role in ER and 37 when it ended. Leaving Carter behind was "melancholy", he says. "It was really hard not to be on the show when it was still on television. I found that very disconcerting." Did he watch it? "Once. It was like watching somebody else raise my kids."

He left the show at the end of 11 seasons, reappearing for four episodes in the 12th and five in the 15th and final season. In between there were films including Pirates of Silicon Valley (1999) in which he played Steve Jobs. The TV movie led to an invitation from the Apple co-founder to appear at the Macworld conference in 1999. Jobs hated the movie but liked Wyle so flew him in as a surprise guest.

"He opened the door to his hotel suite, looked at me and said, 'Yeah. You look like me'…" Then he dressed Wyle up in a black turtle neck, blue jeans and round glasses and handed him a skit to learn. "It was", says Wyle, "a memory I'll take to the grave."

He would have stayed on ER forever had he not had a family: "I saw my son briefly the day he was born and then I went back to work and worked for 16 hours a day all that week. I went back to work the following Monday and for the first time in 11 years I looked at my watch. And I thought, 'Uh-oh, I think there's some place I'd rather be…'"

He sees the rest of the cast occasionally. In April George Clooney (who played Dr Doug Ross in the first five seasons) jokily offered to pay his bail when he was arrested on Capitol Hill at a protest against Medicaid cuts. Clooney had been arrested there a month earlier at a demonstration outside the Sudanese embassy.

"Purely coincidental. I'd promised my friend a year earlier that I was going to take part but when I saw George get arrested, I thought 'Oh shit, this is going make it look like I'm following his heels'. But I would follow him anywhere. He's an incredibly impressive man." In the end, Wyle spent 12 hours in a jail cell. "And it was one of the most rewarding experiences of my life, actually".

He doesn't feel any particular responsibility to use his profile for good – "I think being an actor means being an actor". He has, though, been artistic producer (or chief bankroller) of the Blank Theatre Company in Hollywood for nine years, having appeared in its first show, a "terrible" production of Sexual Perversity in Chicago, 21 years ago.

He hops up on stage ("my gymnasium") occasionally, most recently to play Salvador Dali in an absurdist drama and is filming a couple of indie movies – Snake and Mongoose in which he plays the head of Mattel toys, and Scribble about a squabbling writers' group.

And an ER reunion – could it ever happen? "If the right people came back I probably wouldn't have a problem. But I read one of the last interviews John Lennon ever gave. He was walking down the street and someone shouted, 'When are the Beatles going to get back together?' His retort was: 'When are you going to go back to high school?' Yeah... That feels about right."

'Falling Skies' starts on Tuesday at 9pm on FX UK