The Irish actor’s character Tom Branson has become one of the show’s most important characters.
Allen Leech thought he’d arrived in Downton Abbey for a short-lived stay that wouldn’t do his career any harm.
Instead, Allen’s made the switch from downstairs to above stairs, from chauffeur to saviour of the estate and one of the show’s biggest stars.
Along the way he’s become a heart-throb on both sides of the Atlantic and now has movie producers beating a path to his door.
“When I arrived it was for three episodes,” Allen, who plays Tom Branson, told The Sunday Post as the series gears up for its big finale next week.
“Now I feel really settled as part of a show I love. I’m very happy with where things are with Tom and I’m happy to stay.”
With filming taking up seven months each year, Allen’s other projects have to be slotted in when time allows.
And his new high profile means there’s no shortage of options.
He’s been working on a movie, the Imitation Game, with Keira Knightley and Benedict Cumberbatch, about the cracking of the Enigma Code which helped win the Second World War. Another thriller, In Fear, is out this month.
That bond with the Downton cast is such that they love to shed off the period clothes for a bit of partying whenever they can.
And it can be a starry affair with this series attracting big names, including Oscar nominee Paul Giamatti.
“We had quite an interesting night with Paul when we took him clubbing in Reading,” laughs 32-year-old County Dublin-born Allen.
“We’d gone for dinner with Michelle Dockery, Laura Carmichael and the show’s creator, Julian Fellowes.
“We asked if he wanted to go for another drink and ended up in a club with all this heavy house music and dry ice.
“It was about one in the morning and we could see people at the bar thinking: ‘Am I drunk, or is that Paul Giamatti and the Downton cast?’ Once they’d recognised him people kept hugging him all night.
“There are some pretty decent drinkers on the show. I’d say Elizabeth McGovern is one of those who’s a lightweight in Irish terms anyway but that’s only because she’s a lady!”
Allen’s had the thrill of being at the Emmys when the show has swept the board. And he’s seen the way the Americans have been caught up in the programme.
“I was there just after the episode where Lady Sybil is left pregnant in Dublin and Branson comes over seeking refuge at Downton,” Allen adds.
“I was at Customs and the guy looked at the passport, looked at me and said: ‘That’s a horrible thing you did to your wife’.”
Downton Abbey, ITV, Sunday, 9pm