Allen Leech says the 'Downton Abbey' movie will reduce fans to "tears".
The 37-year-old actor played Tom Branson in the award-winning historical drama series and has teased what fans can expect from creator Julian Fellowes' big screen adaptation.
He said: "It's Julian - you're going to have tears. He's never too saccharine-sweet, so be prepared for everything."
The Irish star also admitted he was "surprised" by the storyline Julian chose, but heaped praise on the Oscar-winner for managing to find a fitting tale for all "22" members of the cast.
Allen said: "I was very surprised about the angle that Julian went with.
"But then again, I was also amazed that he managed to get the entire 22-[member] cast to have their own story within the contained two-hour movie.
"He's done an amazing job."
The 'Bohemian Rhapsody' star says fans of the TV show have no need to worry about how it will transpire from a small screen series to a feature film, as he teased it's one "big, epic story".
According to Variety, he said: "It's a big, epic story that's definitely going to fill the big screen.
"That was our worry about taking the show from the small screen to the big screen.
"But you have Julian Fellowes, who won an Oscar for writing, and he's done a great job with the story."
Screenwriter Julian has already enjoyed success in the movie business, with his 2001 film 'Gosford Park' winning the Academy Award for Best Original Screenplay.
But he admitted that transitioning from TV to cinema does present some big challenges.
He said recently: "[In a series] we usually give the big stories to maybe three characters a week ... By the end of the series everyone would have big stories and it would all be plaited together.
"In a film you can't do that. Every character you include you must have their own story. The viewer can judge whether or not I have been successful, I wouldn't make any claims.
"I had to make sure every story of every character was completed within the movie. That did take a certain amount jiggery-pokery, but I am pleased with where we came to and very pleased with the reassembled cast."
And Julian said reuniting with the cast on the set was a "very happy" moment.
He said: "It was a very happy time for us all. It was a big hit all over the world.
It was a very nice cast and I think they made a lot of good friends among them so there was something of a reunion like the first day back at school ... a very happy school."
Allen and Julian star alongside cast mates including Dame Maggie Smith, Michelle Dockery, Hugh Bonneville and Laura Carmichael in the forthcoming movie, which is to hit cinemas on September 20, 2019.
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