The royal film in the news is The King's Speech, and so – as ever the first with film – I have been catching up with the 2008 version of the novel The Other Boleyn Girl. Tom Hooper's award-winner has been accused of playing fast and loose with historical fact. But the earlier film, starring Natalie Portman and Scarlett Johansson, manages to virtually edit out a rather large historical fact: the Reformation.

Henry VIII is not characterised as Henry VIII at all; he has no Henry VIII-like qualities. He is just a fairytale king in a fairytale story. Figures such as Thomas More have been removed and the story of Henry's divorce from his first queen and its massive historical consequences reduced to an entirely unrealistic trial scene. The dialogue, full of 21st-century banalities, gives no clue that these were serious people in a serious time.

But historical fiction is not historical fact, as any glance at a bookshop would reveal. None of the classics of the genre, from Robert Graves's I, Claudius to Hilary Mantel's Wolf Hall, are pedantically accurate. They are stories. Entertainment. So which liberties are acceptable to take, and which are not?

If you do not find history interesting, it does not matter – the historical fantasy does not have to mean anything or depict anything true about the past. But if you do think historical fiction should deliver some kind of historical meat, it does not have to be served up sedately.

I was surprised that critics were so hard on the supposed inaccuracy of Ridley Scott's Robin Hood. It plays around with the legend of the green man of the forest – which never was a true story – and garbles the story of King John and Magna Carta: but so what? Ridley Scott loves the colours and textures of other times and places. In all his history epics he goes out of his way to recreate not so much the narrative details as the look and heft of history: the design of a medieval siege engine, the rituals of a particular moment. In Robin Hood, there is a shot of London that features a loving reconstruction of the old gothic St Paul's – burnt in the great fire of London – at the heart of what by present-day standards is just a town surrounded by forest. Whatever its faults, it makes you realise that medieval England was a far greener country, with far fewer people, living in a world still dominated by natural cycles and seasons.

For me, that is the kind of insight that history should be about. History is an act of imagination. It is about trying to get inside other people's skins, about seeing the world from remote perspectives. Historical cinema can do this brilliantly, and it does not have to be pedantic to create a sense of time travel. You can read a dozen books about Roman society, but it took Scott's film Gladiator to put audiences into the stone seats of the Colosseum, to share the passions of the raging crowd – to feel, for a moment, the emotions of a Roman.