Naomi Watts spent hour upon hour listening to the Martin Bashir interview with Princess Diana, so that she could perfect that slightly breathy voice and the good news is that she cracked it.
Whatever else anyone says about " Diana", it would be hard to take issue with the choice of British actress Watts to play her - twice Oscar nominated, credible, and beautiful - even though she does not look exactly like Diana, she has studied her facial movements, her walk, well enough to convince.
Why she agreed to play one of the most famous, talked about, photographed and mourned women in history is another matter. She obviously likes a risk - it was the fear of the role that in the end won her over she says - facing the extraordinary challenge.
The film opens at the end - the last few moments of Diana's life as she waits in that Parisian hotel for the car to carry her and Dodi Al Fayed to their tragic fate.
I could feel the sadness moving in - we have seen those images in the lift with Dodi and Henri Paul so many times, and they are recreated here - but then we are taken back two years to the love story that is the centre of this film.
The trailer for this film presented it as a sort of Mills and Boon tale - groan - but the film is better than that thank goodness though at times the dialogue does descend into the schmaltzy - you're so good at giving love oozes Diana's therapist come healer, but you must receive it too.
Diana, based on documentary maker Kate Snell's book Diana: Her Last Love, paints the picture of a woman haunted by the desertion of her mother when she was just a child, deserted by the family she married into, a woman desperately lonely, who only gets to see her children every 4 or 5 weeks and who meets the Pakistani heart surgeon Hasnat Khan on a hospital visit and then falls in love.
But of course with Diana came the madness of a life in the public eye - the pursuits by the paparazzi, and her own collusion at times with the media. Her interview with Martin Bashir is featured here, with Watts brilliantly recreating that look to the camera, as she talks of "three of us in this marriage" and how " it was a bit crowded".
Khan who was with her for two years until weeks before her death is seen as a man who simply found life in the eye of a constant media storm too much, whose own family, though charmed by Diana on one of her visits to them in Pakistan, could not envisage their talented Muslim son marrying a divorced Christian woman.
Enter Dodi, who the film shows as being effectively used by Diana to make Khan jealous. The film ends in the same hotel where it started. Mercifully we are not shown how that car journey ends.
Naveen Andrews, one of the stars of the hit US drama Lost plays Hasnat Khan, the surgeon here. He looks nothing like Khan and indeed the surgeon succeeded for a long time in keeping his relationship with the world's most famous woman private, so Andrews wouldn't have had a lot of material to study to perfect his subject.
Indeed Khan is reported to have said the film is based simply on gossip and on people's testimonies who knew nothing about the relationship. He had no involvement in the project at all he says and he won't be going to see it.
Neither I suspect will Princes William and Harry. They are featured very briefly in one scene in the film, as their mother tells them "see you in four weeks". But of course we know she never did see them again. For Diana's family and confidantes the film will prove a difficult watch - simply because it is weighed down with how we all know it ended.
The director Oliver Hirschbiegel won international fame and an Oscar nomination for his film Downfall depicting the last days in the life of Adolf Hitler.
He will find that recreating the last days of a woman who millions still adore, who feel they know her, indeed own her, will not be as universally well received. Not because of his skills as a filmmaker. But because 16 years after Diana's death, bringing her story to the big screen, albeit a brief section in which she was happy, is still too soon, the wounds still too raw for too many.