Twitter generation beware: Don't expect too much gossip from My Week With Marilyn. Marilyn, of course, is Monroe, and the week is the one spent with her by a then-callow Brit named Colin Clark, whose memoirs of the experience give the script its well-crafted substance plus its respectful tone.

The result plays like a BBC period piece with all the attendant strengths and the usual glaring weakness. The acting is superb, the settings are beautifully recreated, the dialogue crackles with occasional wit, but where's the juice? Although lovely to gaze upon, the whole thing feels a bit precious and porcelain, more teapot than sexpot.

The year is 1956, when Colin (Eddie Redmayne) runs off to London to "join the circus" – more precisely, to work as a go-fer on the upcoming production of The Prince and the Showgirl, a piece of comedic fluff starring Laurence Olivier and the already-famous Marilyn.

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With the new kitchen-sink realism dominating the British stage, poor Larry (Kenneth Branagh) senses that things are passing him by and hopes that his hot co-star will melt the tarnish off his image. Recently married to playwright Arthur Miller, and pair-bonded with acting coach Paula Strasberg, Marilyn is also in the image-changing biz – she wants the artsy respect that comes from hobnobbing with the English theatah types.

That's all familiar lore but, to his credit, director Simon Curtis (yes, a BBC veteran) lays out these separate ambitions and conflicting tensions with breezy dispatch in the early frames. For our part, we're busy measuring the distance between the big names and their impersonators. Not surprisingly, a typecast Branagh makes for a very credible Olivier, capturing the florid self-importance no less than the festering insecurities. And Julia Ormond turns in a wonderfully grounded cameo as Olivier's then-wife Vivien Leigh, who sees her husband far more clearly than he sees himself.

Superb too is Judi Dench's take on Dame Sybil Thorndike, an oasis of kindness on the stormy set, and honest enough to admit to the nervous American: "None of the rest of us knows how to act for the camera – but you do."

Which bring us to Michelle Williams in the pivotal role. Although not as top-heavy as the actual bombshell, she definitely fills out the rest of those iconic gowns, takes care not to overdo the breathy voice, hints delightfully at a quicker-than-expected mind, and fully embraces the little-girl-lost vulnerability.

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Unfortunately, the screenplay gives her no chance to explore the aspect of Monroe that tends to get forgotten: her street-smart, up-from-the-gutter toughness. Too bad because, with her talent, Williams could have done something special with that added dimension – a great actor navigating a great star.

Instead, the movie-within-a-movie sequences unfold as expected. The prickliness is predictable: Marilyn is consistently late and soul-searching ("Acting is the truth"); Olivier is perpetually irked and dismissive ("Acting is faking the truth"). Coddled by her entourage and leaden from drugs, she flubs lines, fights fatigue, wanders off.

Yet, every once in a while, she also flashes the white-heat magic that the camera, and even Olivier, positively adore ("She's pure instinct. She's astonishing"). Okay, but the legend-makers have been beating that drum for decades now – it's common knowledge. What's not is the "week" so provocatively embedded in the title. What about that?

Well, I'm afraid the week is rather weak. Only 30 herself, Marilyn takes a shine to the 23-year old Colin who, as the son of art historian Kenneth Clark, has his own high-placed connections. She invites him to her rented house, shares a few confessions, shows off her sensitive Norma Jean side, and watches his freckled face get completely smitten. To ward off her anxieties, he suggests, "You should see the sights." Deliciously, she snaps back, "I am the sights."

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Turns out they're both right. On their subsequent getaway, he shows her Eton College and Windsor Castle. Then, while skinny-dipping in a local stream, she shows him architecture just as impressive. Later, when Miller has flown the coop ("Why do people I love always leave me?") and her night-demons are on the prowl, the star invites the kid into her bed, where they cuddle, maybe platonically.

Obviously, young Colin is unable to appreciate the lesson that Olivier had learned: "When it comes to women, you're never too old for humiliation." No doubt. But that lesson has a pop-culture variation: When it comes to a woman named Marilyn, it's never too late for veneration. The legend lives on; this pretty film is its amber.

My Week With Marilyn