James, who played Lady Rose, said on Wednesday that discussions over a feature film were "definitely" under way.
The ITV period drama's creator, Julian Fellowes, also said this week that it was "pretty likely" that the film would be produced.
At the Los Angeles premiere of Baby Driver, James told the Press Association: "There's definitely rumours and discussions going on."
When pressed if she was involved in those discussions, she smiled and replied: "Um, maybe."
Fellowes said in a web chat with readers on The Guardian website on Tuesday that he was awaiting the "green light" from a studio.
"I know the cast are behind it and there's a big audience out there," he wrote.
"On most movies, unless it's Spider-Man 6, you're taking a punt.
"But it's pretty likely the Downton movie will happen."
British actress James, 28, stars as the love interest of a getaway driver in Shaun Of The Dead director Edgar Wright's latest film.
Kevin Spacey, Jon Hamm and Jamie Foxx also feature in the star-studded line-up.
James said she is thrilled to work with the British director, adding: "Edgar Wright is such a visionary, I've always wanted to work with him."
:: Baby Driver is out in UK cinemas on June 28.