The Berlin International Film Festival was aquiver with the news. Naomi Watts, it was announced, is set to take the lead role in a Diana, Princess of Wales biopic. She said all the right things: revelling in the honour of playing this tragic figure, and hoping to rise to “the challenge of playing her on screen”. The director of the proposed film, Oliver Hirschbiegel, was keen to note the qualities of a “truly exceptional actress who embodies the warmth, humanity and empathy of such a global icon as Princess Diana” – not least because he’s after plenty of funding for Caught in Flight, before shooting begins, it is hoped, later this year. All interesting stuff. But what has been generally ignored in the excitement is the storyline. Because, apparently, the intention is not to follow her from the gauche young Lady Diana Spencer to her death in a Paris underpass as one of the world’s most famous women. Instead, it will reportedly focus on the last two years of her life, and in particular her relationship with the Pakistani heart surgeon Hasnat Khan. In Hirschbiegel’s hands, this is mouthwatering stuff – he plotted the last moments of Hitler’s reign to marvellous effect in Downfall. But it does raise one question. Whatever happened to the expansive, life-spanning biopic? After all, biographical films chronicling a famous life were once written into the DNA of cinema. David Lean’s Lawrence of Arabia, Richard Attenborough’s Gandhi (and to a lesser extent Chaplin), Spike Lee’s Malcolm X and even Raging Bull – Martin Scorsese’s take on the boxer Jake LaMotta – are all epics in the truest sense of the word. These days, it feels as though there has to be a singalong musical angle for such films to get the green light – as with the life stories of the musicians Ray Charles (Ray), Edith Piaf (La Vie en Rose) and Johnny Cash (Walk the Line). Otherwise, biopics have become focused, lean and concentrated on specific events. Take a look at this year’s Oscar-nominated performances with a biographical bent. My Week With Marilyn might have traded on the legend of Monroe, Michelle Williams looking suitably pouty on the poster. But it does, literally, focus on one week of the late star’s life, when she made the 1957 film The Prince and the Showgirl. The constituent parts of The Iron Lady are closer to the traditional notion of what a biopic should contain, but even this project was more of a portrait of old age and decrepitude. After all, it’s a film about a politician that contains barely any politics, and Meryl Streep appears to lend Margaret Thatcher a sense of humour that no one noticed at the time. Still, the trend towards smaller, self-contained snapshots of specific moments in a life is understandable, if only because it’s far easier to manage from a filmmaking point of view. The King’s Speech, for example, is the only biopic to have won Best Picture at the Academy Awards in the past 18 years, and yet it focuses on a very narrow chapter in the life of Britain’s King George VI. Historians might have been interested to see the very special relationship that developed between George and Winston Churchill, the prime minister during the Second World War, but the film ends as Colin Firth makes his first, stammer-free wartime speech. Nevertheless, it made sense within the constructs of a film which, actually, was more about the relationship between a monarch and a commoner who could “cure” him of his speech impediment. Broadening it into a war drama would have been ridiculous.