there are "new elements" that will interest long time fans.
He added: "For my character, I think the usual [can be expected], which is that he's a sort of dinosaur trying to be led into the future reluctantly, and then eventually he stumbles forward a bit.
"So, as always, there's that rhythm of things changing with glacial slowness, and Robert finally accepting that things [are changing] -- it's time to move the story on, so to speak.
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"It's got the usual tropes, if you like, of thrills and spills in a very Downtown way - which means spilling a tea cup, pretty much!
"I think it'll certainly be the best iteration of the film versions yet."
The Sun revealed that a third movie was in the works in March, with filming starting this summer.
The cast reunited for a read through of the script in May.
In a video taken behind-the-scenes Michelle Dockery, who plays Lady Mary Crawley, opened up about what it was like to see her co-stars again.
She shared: "It feels amazing, a tad emotional. It's wonderful to all be back together again. We come back together and it's like no time has passed at all.
"We're so happy to announce that we're in production for the third Downton Abbey movie. We can't wait to see you."
Joanne Froggatt's return as maid Anna Bates was confirmed by the Sun earlier this month.
A TV insider said: “This will be welcome news for fans of the drama, because Anna was a particular favourite but none of the cast members were guaranteed to return.
“Although the producers have tried to reunite most of the Downton stars, not all of them have been able to commit, even if they did appear in the first two films.”