The spin-off centres around the Crawleys reuniting with their downstairs staff when the King and Queen announce their intentions to stop at Downton for an official royal visit.
Apparently, an early draft of the screenplay would have seen Sue Johnston reprise her role as the Dowager Countess's lady's maid Gladys Denker, but it looks like changes were made during pre-production.
"She was in the first script, then that script disappeared," Johnston has told Radio Times magazine, adding: "Imagine how I felt!"
There's obviously no telling yet why Miss Denker would have been cut. Johnston's former co-star Robert James-Collier has recently described the plot of the reunion movie as "tying up a lot of loose ends".
"Downton is an established brand now, so for me it was a bit of a no-brainer to do a film," he said. "And I'm glad they did."
Although Sue Johnston didn't make it into the first Downton Abbey movie, there's every chance she could return for the sequel – since creator Julian Fellowes is already dropping hints about one.
"I haven't killed off half the cast in a Coronation Street-style crash so a follow-up is a definite possibility," he teased in the spring.
The Downton Abbey film — which brings back Hugh Bonneville, Elizabeth McGovern, Michelle Dockery, Laura Carmichael and Dame Maggie Smith — will be released on September 13 in the UK and on September 20 in the US.
('You Might Also Like',)