Breaking into Highclere as drunk as a lord. Plotting a Downton spin-off with Barrow... set in a gay bar. Playing Cupid to Lady Mary and Bananagrams with the Dowager Countess. And the day George Clooney came to the Abbey. Upstairs or downstairs, Allen Leech has seen it all... as he reveals to Event

‘Dan (Stevens) has done incredibly well. I see him if I’m in America. I can’t say I wouldn’t be excited to work out there but I love being part of Downton, I love being able to do a lot of different work,' said Allen Leech

Lord Grantham should look away now.

Grand illusions will be shattered and high society ideals smashed as Downton Abbey’s favourite firebrand son-in-law spills the beans on the secret life of the stars on Britain’s most popular show.

Allen Leech, who plays former chauffeur Tom Branson in the ITV period drama, is telling me about some very below-stairs high-jinks, such as the cider-drinking session that ended up with him scaling the walls of Highclere in the early hours of the morning to break into his own trailer (‘Thankfully there were no police around’), the time he played Cupid to Michelle Dockery (Lady Mary) by introducing her to the real love of her life; and why the whole cast are lobbying for a steamy affair between Carson (Jim Carter) and Mrs Hughes (Phyllis Logan).

All pretty racy, but just wait till Lord Grantham hears the one about the Downton spin-off Leech has written with Barrow (Rob James-Collier) about the Abbey’s former servants running rival bars (one gay bar, one straight) in New York.

Leech, 33, has the unmistakable sheen of a man whose life could not get much better.

For an actor, being in Downton – with its six Emmys, two Screen Actors Guild Awards, two Golden Globes and ten million viewers – gets you noticed.

He has already starred in one of this year’s most high-profile movies, The Imitation Game, alongside Benedict Cumberbatch and Keira Knightley.

‘You’ve got to just throw yourself in and make it work,' said Allen

And when we meet in a bar near King’s Cross station, he is about to board a train to Leeds to start filming alongside Avatar’s Sam Worthington in Hunter’s Prayer. Yesterday he was in Germany. Leech is living with a bag permanently packed.

‘Downton is massive,’ says the Dublin-born, theatre-trained actor.

‘People you don’t expect watch it. Directors will tell you they love the show. It is an incredible thing to be part of.

'You have these surreal moments, like sitting at the SAGs [Screen Actors Guild Awards in 2013] and seeing Steve Buscemi nodding across the room at you, and then being stopped by Jeff Daniels to be told he likes your work.

'None of us thought we were in with a chance of winning anything, so in the cab on the way over we drew lots on who would speak.

'Phyllis Logan offered to do the cast speech on the basis that it was never going to happen, so when they called our name [nominated in the Ensemble category; Maggie Smith later won as best female actor] she looked at me and mouthed a few expletives that would never be heard on Downton.

‘As we stood on the stage, I was eyeballing Robert De Niro, who was standing in the front row grinning and clapping, then backstage Daniel Day-Lewis walked past and said: “Great stuff, Allen. Doing it for the Irish.” It was like being in a dream.’

Leech spins a cracking yarn. Like the one about the night he and Rob James-Collier stayed up until the early hours of the morning drinking cider.

‘We’d been at work so we decided to have a few beers followed by quite a lot of Old Rosie cider.

'We were staying in different hotels and unfortunately I’d forgotten to check in to mine; the doors were locked and I couldn’t get in.

'I called Rob but he was out cold, so the only option was to go back to the set and try to sleep in my trailer.

'I used my bag to cover the spikes on the massive gates as I shimmied over, then dodged through the darkness to the trailers.

'The next morning I had a two-hour walk back to my hotel to clean up and come back. I then had to do a very long scene with Richard E Grant with the footsteps of Old Rosie banging through my brains.

'Thank God I wasn’t arrested, as that would have been very embarrassing – Branson breaking into Downton.’

Leech’s ability to tell a tale helped him land the Downton role. The show’s creator, Julian Fellowes, cast him for his World War II movie From Time To Time (which also started Downton’s Maggie Smith and Hugh Bonneville) in 2009.

They kept in touch because ‘we both started off in the theatre, we both like telling stories and we both love a good chat’, and Fellowes made sure he got his man for Downton.

‘Downton (Abbey) is massive. People you don’t expect watch it. Directors will tell you they love the show. It is an incredible thing to be part of,' said Allen

Leech is a team player. When it came to Event’s Christmas cover shoot, he gamely laced up his boots and flung himself around the stunning ice rink in the grounds of London’s Natural History Museum, posing confidently for the camera and exchanging banter with the fans who made it past security.

‘You’ve got to just throw yourself in and make it work,’ he says, succinctly expressing his life’s philosophy.

‘My last movie [the Sundance-acclaimed chiller In Fear] was filmed over weeks of night shoots in a wood in Cornwall.

'At one point I was wrestling in wet mud in sub-zero temperatures from 3am to 8am to get a few minutes down on film.

'We had one break in the middle and when they wanted to get going after two minutes I did, for the first time, just shake my head and say, “I need a minute longer.” But you can’t be a diva.

‘When I worked on The Imitation Game with Benedict and Keira we filmed a lot of it in a freezing warehouse with just one old toilet. No one moaned.

'On Downton we get very well looked after, which is part of the reason everyone loves working on that show.

‘Benedict and Keira were great to work with – again, it was a team.

'Benedict and I have friends in common so we felt we’d known each other for ages, and Keira is just a lovely woman.’

It took less than one series for Fellowes to decide he wanted the ‘temporary’ character of Tom Branson to go ‘main’ (such is casting lingo).

The writer conjured up an astonishingly dramatic (even by Downton standards) series of plotlines, from his affair and subsequent marriage to Lady Sybil (played by Jessica Brown Findlay), a brief sojourn as a struggling couple together in Ireland followed by pregnancy, death in childbirth and a new life for heartbroken Tom upstairs with the Crawleys as the forward-thinking estate manager of the family.

On screen he is eternally conflicted, between his socialist beliefs and his upper-class status, his recent half-hearted romance with the outspoken schoolteacher Sarah Bunting (Daisy Lewis) and his undying love for Sybil, and between setting up a new life running a car business in America or staying with his daughter in the bosom of her aristocratic family.

‘That’s Branson,’ grins Leech. ‘Doomed to sit on the fence in life.’

The Christmas Day Downton is the bookies’ favourite to be the most-watched TV show this season, with the promise of several big cliffhangers ready to be resolved.

'George (Clooney) loves the show. He’s an incredibly relaxed actor, which meant no one around him got nervous. He was just there for the afternoon, everyone had a chat with him,' said Allen

A Crawley family trip has become a Christmas tradition and this year the family is on the move again, to another historic home (the cast were spotted filming at Alnwick Castle, in Northumberland) where they are to be entertained by Lady Rose’s (Lily James) in-laws, Lord and Lady Sinderby (James Faulkner and Penny Downie).

The perennial sub-plots remain. Can Lord Sinderby ever be pleasant to Rose? Will the Dowager get over the fact that the Sinderbys are Jewish, and will Isobel Crawley (Penelope Wilton) get back on track with Lord Merton (Douglas Reith)?

And what about the big burning questions to be answered in this special?

The juicy stuff we all want to know. Like... did Anna (Joanne Froggatt) murder the rapist valet Green (Nigel Harman)? And if not, will she get pregnant by Bates (Brendan Coyle)? Is Tom going to hightail it to America and leave the Crawleys far behind? Will Violet (Maggie Smith) take up the romantic proposal from her old flame, the Russian refugee Prince Igor (Rade Serbedzija)? Will Carson (Jim Carter) and Mrs Hughes (Phyllis Logan) ever get it together, and who will Lady Mary fall for next?

Leech, ever the team player, is sworn to absolute secrecy.

‘It’s a good one,’ he says about the special, tantalisingly.

‘And by the end everything will become clear. But I absolutely can’t say anything more. I’m bound by the vows of Downton.

'We say nothing. We don’t tweet, we don’t slip up. There was a moment during filming last series when Richard E Grant [who played the art dealer hoping to seduce Lady Grantham] got out his phone and was about to tweet when he had to be stopped.

'It’s hilarious. Dan [Stevens] was the worst. He tweeted that there would be a Christmas special before it even happened. That was big trouble.’

Such is the transatlantic pull of the series these days, this year will be remembered as the one when George Clooney came to Downton.

A special Text Santa spin-off comedy sketch, featuring the world’s most famous film star and co-starring Leech, will be screened on December 19.

'In America the most popular request is for myself and Lady Mary to have an affair, but that wouldn’t be right. The big problem for Branson is that no woman is right,' said Allen

‘Hugh Bonneville [Lord Grantham] set up the whole thing because they worked together on Monuments Men and got on incredibly well,’ says Leech.

‘But none of us actually knew it was happening until the day he walked on set. We’d been told it might happen, so everyone was already excited, but then he was just there.

‘Joanna Lumley is in the sketch too, which was another big thrill for me. It was a pretty special moment.

'George was great. He loves the show. He’s an incredibly relaxed actor, which meant no one around him got nervous. He was just there for the afternoon, everyone had a chat with him – but it was more him asking us about the show than us asking him questions…

‘He is very charming and low-key, and for the few hours of filming we were all in this little Downton bubble.

'Then he got into his car and everyone stood outside the house [Highclere] to wave him off, and as soon as his car disappeared up the road about 700 paparazzi went in hot pursuit, roaring through the village.’

While careful not to reveal too much about what happens when Clooney steps into the Downton drawing room, Leech is deliciously indiscreet when I pump him for more gossip on the cast.

Who’s the actor least like his screen character? He laughs.

‘No question. It has to be Rob James-Collier (Mr Barrow). He’s great fun and definitely the wild man of Downton.

'On our tours of America [the cast fly over for regular promotional trips] Rob insists on taking a drinking buddy.

'A lot of the actors are very sensible, but Rob likes to be up all night having fun.

'On that level I can match him drink for drink, but I’m getting a bit older and a bit more sensible and my body can’t take it now I’ve hit 33.

‘He also has a huge gay following, which he loves. Downton fans cross all barriers.’

‘When I worked on The Imitation Game with Benedict (Cumberbatch) and Keira (Knightley) we filmed a lot of it in a freezing warehouse with just one old toilet. No one moaned,' said Allen

The camaraderie between Downton stars has become the stuff of acting legend. In between takes they play Bananagrams (‘Maggie will sometimes utter an answer as we’re waiting to speak, which is hilarious – Bananagrams is the favourite cast game’), they eat together, drink together and while the younger cast bunk down in one of the Highclere hotels, the older ones stay at a more formal location, with Dame Maggie residing in a rented house.

‘I think we spend more time together than most casts,’ he says. ‘There is a sort of hierarchy, which works well.

'Hugh is the daddy, Maggie is the dame – we all know each other very well and everyone is incredibly supportive. It is the most genuinely happy team.’

Like the rest of the cast, Leech has his favourite fantasy plot twists.

‘I think the most popular one is that Carson and Mrs Hughes have an affair. We all want that. Times have moved on. It could happen.

'In America the most popular request is for myself and Lady Mary to have an affair, but that wouldn’t be right. The big problem for Branson is that no woman is right. I can’t think of a single member of the cast he could be with.’

The spin-off series he has co-written with James-Collier has been, he says, touted around several agents. ‘It’s basically Barrow and Branson running rival bars in New York, Cheers meets Downton. We’ve showed it to Julian but he’s a bit non-committal. I think it’s a great idea.’

On girlfriend Charlie Webster: 'She’s a woman in a man’s sport and she was totally fearless,' said Allen

Leech’s own life could not be more different to Branson. He doesn’t own a car (but hankers after a Moto Guzzi motorbike) and gets around by taxi.

The son of middle-class Irish parents – his dad, David, is CEO of a computer company – he studied drama at Trinity College, Dublin, but spent most of his time as a student working or going to auditions.

For the past year he has been dating Sky Sports presenter Charlie Webster, who last month hit the headlines when she resigned her post as patron of Sheffield United football club after convicted rapist Ched Evans was allowed to return to football training. The offer to Evans was later withdrawn.

‘I was in America on a promotion tour of The Imitation Game when it happened,’ says Allen.

‘But we spoke all the time and then she flew out to be with me.

'I was incredibly proud of her. She was the first to stand up and be counted. She’s a woman in a man’s sport and she was totally fearless.

‘I love that about her. She was sticking up for what she believes is right, but she got massive abuse on Twitter and from some of the fans, really vile stuff that was just absolutely disgusting.

‘She is a gorgeous woman but she’s more gorgeous because she has strong principles and beliefs and I’m lucky to have found someone like her.’

He reveals he has also played Cupid to his fellow Downton star, Michelle Dockery, who is dating his former flatmate, financial PR John Dineen, a friend of Leech’s from home in Ireland.

‘John’s like family to me and I knew he’d really get on with Michelle. She’s a great girl – probably as far from Lady Mary as you could imagine.

'Mary can be an absolute bitch, but Michelle is sweet, down to earth and funny.

'There’s been many a time we’ve been out together drinking pints of beer and people come over and do a double-take – especially in New York, where a lot of people genuinely believe you are those characters.

‘[The cast] are now part of my extended family. All of us get together every now and again and I’ll ring Hugh or Jim [Carter] or Penny [Wilton] for advice if I’m offered a role.’

Of his own storyline, Leech remains tight-lipped. Odds are he will be following in the footsteps of Dan Stevens and beating a retreat to Hollywood.

‘Dan has done incredibly well. I see him if I’m in America.

'I can’t say I wouldn’t be excited to work out there but I love being part of Downton, I love being able to do a lot of different work.

'It was actually great fun to play a complete psychopath in In Fear, with all that blood and guts, because you couldn’t get further from Branson,’ he says.

‘All will become clear at the end of the Christmas special.’

The large bag he has with him by his side is packed. This could be the real clue. Expect tears in the Abbey.

‘Downton Abbey’ is on Christmas Day at 9pm on ITV1.

Photographs taken at the Natural History Museum Ice Rink, London. For tickets, go to nhmskating.com