When the first images were released of Michelle Williams in character as Marilyn Monroe last week, the reaction wasn't just one of amiable interest. It was ecstatic. The Shutter Island actress plays the blonde bombshell in Simon Curtis's directorial debut My Week with Marilyn, out next year - the promo shots prompting the Daily Mail in the UK to coo that she looked "uncannily like" Monroe. Grazia magazine said she looked "so amazing our heads might explode". Even Perez Hilton - not exactly the friendliest gossip columnist in the blogosphere - said she looked "incredible".
Take a closer look at the picture, though, and it's actually a triumph for the make-up team, of well-styled hair and a correctly placed beauty spot. Michelle Williams, when she walks down the street, doesn't look anything like Marilyn Monroe. But making her seem like she could is a vital element of this film's potential success. Or is it?
Casting a lead for a biopic where the subject is recognisably famous - whether that be Will Smith for Ali or Denzel Washington for Malcolm X - is admittedly one of the hardest decisions a director and his studio have to make. And one of the most difficult tasks an actor will face. Not only does the actor in question have to have something about them that make-up artists will be able to turn into a passable impersonation, they have to walk and talk like their real-life subjects to entice people into the cinema.
That's the common consensus, anyway. But though Naomi Watts has been chastised in some quarters for not having enough curves for the second Marilyn Monroe film currently in development (Blonde, the adaptation of Joyce Carol Oates' book), it's not just about her body shape, in the end. The best biopics are those that cast the actors who are capable of capturing the essence and beating heart of their subjects, rather than their looks or the exact way they talk. Otherwise, every two-bit impersonator on a television sketch show would have had their shot at the Hollywood big-time by now.
Acting, then, is far more important than accents. Take Michael Sheen. Nobody in their right minds would suggest that Kenneth Williams, Tony Blair, David Frost and Brian Clough looked or spoke anything like each other. Yet Sheen has played them all with aplomb in Fantabulosa!, The Deal, The Queen, Frost/Nixon and The Damned United. The voice and the mannerisms might have been spot on each time, but these aren't just clever impersonations in different costumes. Sheen gives these characters a soul and a narrative of their own, and one that perhaps stands outside the general perceptions of what we might have had of the famous people he plays.
Amplifying these characters rather than playing them straight is key. Josh Brolin's George W Bush in W was something of a comic creation - and all the better for it. Joaquin Phoenix inhabited Johnny Cash to such an extent that he sung the songs himself in Walk the Line. And it gave the film a life and vitality missing, to an extent, from the Edith Piaf biopic La Vie en Rose - in which Marion Cotillard mimed the real recordings. True, Jamie Foxx also mimed his way to an Academy Award for Ray (about the soul singer Ray Charles), but there's something far more thrilling about Sam Riley actually singing the Joy Division frontman Ian Curtis's lines in Control, and Andy Serkis ranting his way through the Ian Dury biopic Sex & Drugs & Rock & Roll.
In fact, Michelle Williams would do well to note that all of these films are successful because they're not completely devoted to their subjects - they represent a life rather than replicate it. Morgan Freeman has been Nelson Mandela-in-waiting for years now, thanks to a striking physical resemblance and in no small part to an invitation from Mandela himself. Freeman isn't bad in Invictus, but the film itself is overly reverential. The same problems bedevilled Hilary Swank's turn as the pioneer pilot Amelia Earhart in Amelia and Kevin Spacey's impression of Bobby Darin in Beyond the Sea. Still, they weren't as bad as Oliver Stone's Doors biopic. Val Kilmer actually does a pretty good job with what he's given. But, as the celebrated film critic Roger Ebert said at the time: "Having seen this movie, I am not sad to have missed the opportunity to meet Jim Morrison, and I can think of few fates more painful than being part of his support system."
So it will be interesting to see how Al Pacino approaches the life of Phil Spector - according to The New York Times this week, HBO Films plans to give the music producer, currently serving a life sentence for murder, the biopic treatment. Interesting not least because Al Pacino is 70 years old, and Spector was in his twenties when he shot to fame with The Ronettes' Be My Baby.
That'll have to be some wig.