The public's taste for life-stories is insatiable; people always want to read about people. That's why biographers are among the highest paid in the literary world, why Hollywood is so fond of biopics – only this week producers began searching for an actor to play the astronaut Neil Armstrong – and why TV and the theatre is turning increasingly to the lives of long dead writers, politicians and even comedians.

On the page, a biographer can get away with a paragraph or two to describe someone's walk or bearing, manner of speech or a specific mannerism. Photographs also serve as an aide-memoire to whoever buys the book. But on the screen or stage, the actor has to vividly recreate, and in fact almost impersonate, whoever is being portrayed to make them credible and flesh again.

Right now the National Theatre contains two perfect examples. Jeremy Irons is a very believable Harold Macmillan in Never So Good. The tall, broad-boned Vanessa Redgrave, despite being the physical opposite of the bird-like and delicate Joan Didion, becomes her nonetheless in The Year of Magical Thinking. The difficulty occurs when the intelligence is meagre. Armstrong will be a hard act because he is reticent and private and doesn't give himself away easily.

And take the case of George Orwell, who ought to be ripe for the movie treatment. Orwell worked for the BBC between 1941 and 1943, and yet no recording of his voice survives. The only film of Orwell – well, it's almost certainly him – is over in an eye-blink. He is strolling, hands in pockets, down a street in Suffolk. What did Orwell sound like? As he went to Eton, smoked like a laboratory beagle and took a bullet in the throat in Spain, it's probably safe to assume he was fairly posh and the words rolled out of his mouth like car tyres over gravel. How did he move? The short clip shows a tall, immensely straight back-man, like a walking hat stand. How did he express himself? His prose is clear, perfectly clipped and stoic, and he was probably much the same in conversation. But when the West Yorkshire Playhouse put on Year of the Rat last month, in which the tubercular Orwell writes Nineteen Eighty-Four in his lonely farmhouse on Jura, the play struck me as a noble failure. Orwell just wasn't Orwell; at least not the Orwell I expected from the journalism he wrote and the evidence of his biographies. Strands of his hair flopped forward, as if he was Hugh Grant in Four Weddings and a Funeral. He slipped into lachrymose histrionics. Orwell was a complicated, puzzling, half-elusive figure and it requires a subtle performance to properly capture that fact. I suspect that the portrait of Armstrong will need to be similarly nuanced to convey him properly. The odds of achieving it successfully may be as long as... well, landing on the Moon, perhaps.