Jeremy Swift, who played butler Septimus Spratt, told ITV's Lorraine that the cast had been sent a script - but that it had since "disappeared".
He went on: "It's supposed to be happening - filming - this year, but it hasn't been locked down yet."
The period TV drama, which was a hit around the world, ended at Christmas 2015 after six series.
There has been much speculation about a movie version.
Swift said: "There is a film script which we've all been sent but it disappeared in a Mission Impossible-stylee from our emails. With a little puff."
He said "logistics" were causing a problem with filming and that Downton's success had meant that some cast members had "flown off into the world" to work on other projects.
"It's just getting everybody in that same space and time," he added. "I think there is a huge appetite for it."
Carnival Films, which produces the drama, has already confirmed that a script for a potential movie is in development.
It told the BBC on Monday: "There are still no firm plans about when a film might go into production.  When the future plans are certain we will of course make an announcement at the relevant time."
In April 2016, Downton creator Julian Fellowes said he was already thinking about a plot for a film adaptation to avoid being "caught on the hop" if it was given the go-ahead.
Downton Abbey received a special Bafta tribute in 2015. It is the most nominated non-US show in the history of the Emmy Awards and was shown in more than 250 territories worldwide.
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A big screen version of Downton Abbey could be filmed this year, one of its cast members has said.