Why do "factual" feature films often tell lies?

I'm grateful to Simon Hoggart of the Guardian (see below) for pointing out that many "factual" elements in films are pure fiction. Sometimes, of course, events have to be conflated and simplified in a feature film, but that is no excuse for gratuitously creating myths, some of them dangerous, that run counter to the truth.

Apparently Saving Mr Banks, the Disney film about the making of the Disney film Mary Poppins, is wildly inaccurate. The prickly relationship between Walt Disney and the author, PL Travers, did not end in kisses and hugs. She hated the film, and especially loathed the Dick Van Dyke part.

But Disney has form. In White Wilderness (1958), he perpetuated the old myth that lemmings commit mass suicide and staged the event. While we're at it, even Cinderella's slipper was probably not glass, or "verre", but fur, or "vair", which seems more likely.

And in other studios, the Americans broke the Enigma code, Braveheart was a noble freedom fighter, and in Argo the Brits refused to help refugees from the Tehran embassy siege. All rubbish.

In the film version of my book God's Triangle it is inevitable that many scenes will have to be guess-work. Also, no-one can be exactly sure what the main characters said to each other as the scandal unfolded. But that said, I want God's Triangle, currently at the pre-production script development stage, to be true to the spirit of my book, a true story.

It was for this reason that I strongly objected to a proposal by a young (male) director that Olga Johnston would discover the truth about my great aunt Florrie Cox when the two of them had a bath together.

Why did I object? Because both women were missionaries and the event was to have taken place in 1917 or thereabouts. I told the producers that it was ridiculous to suggest that any women back then -- let alone sexually-inhibited Christian missionaries -- would have seen each other naked and casually shared a bath. To include a scene like that would undermine the credibility of the entire film.

The producers agreed and the director went off to look at other projects.