Staunton, an Oscar nominee for her performance in Mike Leigh's Vera Drake, is best known for her role as Dolores Umbridge in Harry Potter and the Order of the Phoenix. Staunton's real-life husband is Jim Carter, who plays Charles Carson.
The movie's other big Potter connection is Maggie Smith. She played Professor McGonagall in the Harry Potter films and will reprise her Downton Abbey role as the Dowager Countess Violet Crawley.
"Since the series ended, fans of Downton have long been waiting for the Crawley family's next chapter," Focus Features chairman Peter Kujawski told PEOPLE after Smith joined. "We're thrilled to join this incredible group of filmmakers, actors and craftspeople, led by Julian Fellowes and Gareth Neame, in bringing back the world of Downton to the big screen."
According to Deadline, the other new actors joining the film are Geraldine James (Sherlock Holmes), Simon Jones (The Hitchhicker's Guide to the Galaxy), David Haig (Florence Foster Jenkins), Tuppence Middleton (The Imitation Game), Kate Phillips (Peaky Blinders) and Stephen Campbell Moore (The Bank Job).
Michelle Dockery and Hugh Bonneville are both expected to play their roles from the series.
Focus also announced a director change. Brian Percival, who directed the original Downton Abbey pilot and the film The Book Thief, was originally going to direct. He will instead executive produce with Nigel Merchant. Michael Engler, who directed three episodes of Downton Abbey and the 2015 Christmas special, will now direct the film.
Downton Abbey creator Julian Fellowes wrote the movie. Fellowes won an Oscar for his Gosford Park (2001) screenplay and won two Emmys for Downton Abbey in 2011.
Downton Abbey originally ran for six seasons on ITV in the U.K. and PBS in the U.S. It was an nstant success, earning three Golden Globes and 15 Primetime Emmys. At the Emmys, it was the most-nominated show made outside the U.S. with 69 total nominations.
The film will be produced by Carnival Films, which also produced the series. Fellowes, Gareth Neame and Liz Tirbridge are the producers. Focus Features and Universal Pictures International are handling distribution.
It's not clear what the plot for the Downton Abbey movie will be, or when it will be released.
Fans were excited to hear about the new casting, especially with Staunton joining.
"Imelda Staunton is joining the #DowntonAbbey movie," one fan wrote.
"The Downton movie (still asking myself why though?) cast will be joined by Geraldine James & Imelda Staunton. & that's simply splendid - had me expressing my delight rather loudly for a second there," another added.
The Downton movie (still asking myself why though?) cast will be joined by Geraldine James & Imelda Staunton. & that's simply splendid - had me expressing my delight rather loudly for a second there.— SarLei (@SarLei94) August 31, 2018
Photo credit: PBS