Jared Harris meanwhile, who stars as King George VI, also says he gained a new sympathy for his character, thrust on to the throne following the abdication of his older brother.
"He was someone who didn't appear to be naturally suited to it," Harris says. "His brother was sort of a movie star, a gregarious charming person who everyone thought – quite similar to Margaret – would be better suited to the presentational aspects of the role," he adds.
Princess Diana, photographed in November 1992
"I suppose he surprised everybody by knuckling down and taking it seriously as a sacred responsibility, but it doesn't seem like he ever felt as though he was successful in the job. There was always a feeling of, he was second best."
King George, says Harris, was "imprisoned" by the role – and so was his daughter, according to Kirby. "Margaret too, because in a way you're imprisoned by not being the Queen. You're defined by the fact that you're not.
"And so there's always a sense of lack, I think, and I think what I loved about her so much was this juxtaposition between being the gregarious, confident, extrovert exhibitionist rogue one, combined with a low sense of self-worth."