Marilyn Monroe's appeal can seem hard to understand today. Her strange mixture of innocence, voluptuousness, dumb-blondeness and little-girlishness are as alien to contemporary sexual tastes as the heavyweight beauties in baroque paintings. Nevertheless, the doomed, damaged, superhumanly glamorous star remains an object of fascination almost five decades after her death.

Monroe's latest screen reincarnation is based on the memoirs of Colin Clark, who was an assistant on The Prince and the Showgirl. This was the movie that brought Monroe to Britain and into an impossible professional relationship with Sir Lawrence Olivier, who both directed the comedy and starred in it.

Clark's books recount the tensions and misunderstandings between classically trained Olivier and Monroe, with her "method" training, her unprofessional lateness, and the drugs she used to keep her unhappiness and insecurities at bay.

Clark was starstruck by Monroe, who in turn was thrilled to find a sympathetic Englishman on set, and the 24-year-old youth and the 30-year-old recently married megastar spent a mostly innocent but erotically tinged week together in the English countryside in the middle of the troubled production.

It is a fascinating piece of movie history. Directed by Simon Curtis in his feature debut, it makes for an amusing, occasionally funny, occasionally moving little film. I say "little" film because all too often it looks and feels like a television drama. Curtis employs relentless TV-style close-ups and Adrian Hodges's screenplay lumbers from one cliché to the next. It is a shame because My Week boasts a superb central performance by Michelle Williams (pictured).

The provincial feel of the movie is compounded by the apparent failure of the filmmakers to hire a dialogue coach. All the American characters except for Williams's Monroe are played by Brits and, with the exception of Zoe Wanamaker's Paula Strasberg, the accents affected by the cast are inaccurate and crude (Dougray Scott's efforts to sound like Arthur Miller are simply painful).

And the problem is not confined to the American voices. No one seems to have told Eddie Redmayne (who plays Colin Clark) that, in the 1950s, upper-class young people sounded posher than they do today, and that an Old Etonian would not have had a trace of "mockney".

Redmayne's generally weak performance is made even more obvious by how good Williams is at portraying Monroe's vulnerability - and at mimicking her voice. You can almost overlook what is problematic about her casting. It is not just that she is too skinny to resemble Monroe; she fails to capture the star's amazing sexual glamour. You get no sense of just how powerful that allure was, and how confidently Monroe deployed it.