In an age of tiresome television revivals and reboots, “Downton Abbey” is the exception, returning larger and more lavishly than ever — on the big screen. Opening nationwide this weekend is Michael Engler’s film, written by Oscar winner Julian Fellowes (“Gosford Park”), who created the show that became the biggest hit in PBS history.

Over its six seasons, the escapist upper-crust costume drama spanned the 1912 sinking of the Titanic, World War I and the 1918 flu epidemic. Between these historical benchmarks, the show satisfied an appetite for a romantic drama in which women were treated with respect. At the same time, it offered a bird’s-eye view of the British aristocracy in a time of great transition.

Not surprisingly, the TV show is a hard act to follow. Fellowes and Engler had to pull out the stops for the two-hour feature film. Hugh Bonneville, who plays Robert Crawley, Earl of Grantham, says the movie was fashioned “with an eye toward our loyal audience.”

Engler says the movie needed a single event that could “pull everyone in and raise the stakes for them all.” That event became the arrival of the British King George V and Queen Mary (Simon Jones and Geraldine James) to Downton. “Everything that has to happen around a royal visit gave us [story] opportunities,” Engler says.

While Kate Middleton, Duchess of Cambridge, visited the set of the TV series, the movie boasts an actual royal presence on-screen: The King’s Troop, Buckingham Palace’s own ceremonial team of horses, are shown parading through Lacock, Wiltshire, a National Trust village, to celebrate the royals’ arrival.

“It was just amazing that they would come and play with us,” Bonneville, 55, tells The Post. “We had them for two or three days. The horses are color-coded. They have the darker horses for the funerals, the lighter horses for the weddings. It’s extraordinary. Each of the gun carriages they tow has a plate of what events they’ve been involved in. So there is Lady Diana’s gun carriage and the wedding and so forth. There is history right there passing in front of you.”

Costume designer Anna Robbins went all out when it came to dressing the cast for dinner parties and balls. Check out the cornflower blue dress Lady Mary (Michelle Dockery) wears to the royal repast in the Downton dining room. “The house of Fortuny designed that dress specifically for the movie,” Dockery, 37, tells The Post. “It was a traditional Fortuny dress and fabric. It’s absolutely stunning. That’s when you know you’re doing film.”

The ensemble cast of TV’s “Downton” became larger for the movie, with the addition of small roles by such British theater greats as Imelda Staunton, the real-life wife of “Downton” stalwart Jim Carter, who plays the butler, Carson, and Mark Addy (“The Full Monty”). “The pressure was on the crew, the design team and the cinematographer to make it look and feel more epic, which I think it does,” says Bonneville of the 10-week shoot. (By contrast, the TV show packed seven hourlong episodes into a 20-week shoot.)

“We just needed to turn up and play the roles we played,” Dockery says. “Everything was slightly heightened doing the film. We’d never seen Downton look like that. All that drone work. That first shot comes over the house. We had never seen anything like that on the TV show.”

In “Downton Abbey” the film, everyone downstairs is in a tizzy over the royal visit. That excitement turns to bitterness when Mrs. Hughes (Phyllis Logan) and her crew are told that members of their household staff will be supplanted by employees of the palace.

Upstairs, Lady Violet (Maggie Smith) is steaming like a tea kettle over a potential upset in the line of succession at the estate.

“She feels the house should go to me, as her rightful heir,” says Bonneville, referring to his fictitious earldom. “Maud Bagshaw [Staunton], a cousin, has a different idea.

“As it happens, Robert is perfectly happy not having that inheritance. Violet wants to keep it in the family. What she doesn’t know is that there is another element of family already in play.”

Hovering over the story is the pressing question of what to do with an estate as grand as Downton. With the Great Depression looming, the aristocracy stands to lose everything. Yet, Lady Mary willingly assumes the duties of overseer, an about-face from the days early on in the series, when Mary told her mother (Elizabeth McGovern) that she didn’t want to become the lady of the house.

“I thought she might end up being the rebel,” Dockery says. “But Downton became Mary’s life, and she became more invested in her duty as she was growing up. Downton is the other love of her life, and I don’t think she could leave. It’s the people — they’re her family: Carson and Anna [Joanne Froggatt] and everybody who works for the Crawleys. The Crawleys provide a life for everyone.”

With the “Downton Abbey” film off to a running start — it grossed $12 million at last week’s European release — there’s already talk of a sequel. “We’re back in the company of friends. As long as we can keep Julian alive,” says Bonneville of the 70-year-old screenwriter, who is also writing “The Gilded Age,” a new HBO series. “And prop him up with his pen.”