We had chosen to set our film in November 1932 because I wanted shooting, Altman didn’t want Christmas, and neither of us wanted the Nazis. Since they burnt down the Reichstag in February 1933, this seemed like the last time it would be believable for people to get through a weekend without ever mentioning them.

And it turned out that, at this moment, Novello’s career was in ­crisis. His attempt to remake The Lodger, one of his biggest silent hits, as a talkie had flopped at the box office, and he thought his great years were behind him. In actual fact, the coming decades would make him a giant star of his own operettas, but his despair at the end of 1932 meant the ­character would be a thousand times more interesting, and later, when Jeremy Northam played him so movingly, he became one of the mainstays of the picture. At any rate, by the time I came home a week later, one thing had changed. I was convinced we were actually going to make the film.

In those days, as a writer, I was a hired gun. I had no status and, when it came to casting or anything else, my opinion might be sought, but only as a courtesy. They were not obliged to ask me anything. So it was with a mounting degree of wonder that I followed the announcements of who was to play which part in Gosford Park. Maggie Smith, Helen Mirren, Michael Gambon, Eileen Atkins, Kristin Scott Thomas, Emily Watson, Alan Bates, Derek Jacobi, Richard E Grant, on and on came the names of the best actors in Britain. I was amazed.

Then there was the location. It will not perhaps come as a surprise that I wanted Highclere Castle, a house with which I was already rather in love, and which would serve me well as Downton Abbey. But Altman told me once that he liked to employ stars to play even quite minor parts if he could, as they made the story much easier to follow for the audience, and this could only happen if the stars in question were able to sleep in their own beds. So we settled on Wrotham Park, a lovely house in Hertfordshire, just outside London, and we began.

The budget, at around $20 million, was adequate but tight. It was a very large sum to me, of course, but not when it comes to film-­making. The Young Victoria (2009), which I wrote, cost almost twice that, and The Tourist (2010), another film I worked on, cost five times as much. So it was decided that they would decorate the bedroom of the Countess of Trentham first and shoot in it for a week, while they got the rest of the house ready. 

And that was how we began, a week of scenes with the great Maggie Smith. I know now that, as a rule, Maggie does not need explanations. She understands her lines without difficulty, and can always tell if there is any humour in them. But we did not know each other then and, one day, she asked me why Lady Trentham was disappointed to find bought marmalade on her breakfast tray. I explained that a great-aunt of mine always believed that if any house ran out of its own jams and jellies, then either the cook, the housekeeper or the still-room maid did not know what they were doing. “Got it,” said Maggie, and she effortlessly delivered one of the film’s funniest moments.