1946. Juliet Ashton (Lily James — Cinderella, Baby Driver) is a successful writer lauded for her popular series of comic essays. Restless for her next project and haunted by losses in the recent war, she receives an enigmatic letter from Dawsey Adams (Michiel Huisman — Game of Thrones, The Age of Adaline), a lonely farmer on the island of Guernsey.

Juliet learns of the titular society set up by accident during the Nazi occupation and Dawsey’s tales of life during wartime so intrigue her that she journeys to Guernsey. There, she becomes involved in the lives of the idiosyncratic characters of the society, and discovers a story of friendship, bravery, and tragedy that will ultimately change her life.

Pity the writers who must adapt a beloved, best-selling, epistolary novel for the screen. Mary Anne Shaffer and Annie Barrows’s novel is so defined by the letters written by its characters — and so emotionally bound with the tone of those letters — that it was almost always going to be impossible to adapt it well for film.

Happily, Thomas Bezucha (The Family Stone), Don Roos (The Opposite of Sex), and Kevin Hood (Becoming Jane) do a fine job of getting the characters tonally right, and the film is charming and poignant but not without its faults.

There’s something inherently wrong in the casting of the two leads. Lily James has bright-eyed spirit but lacks the vital spikiness of the novel’s Juliet, which made her so resolute. I don’t for a second believe she is a writer. She lacks gravitas, something a more involved backstory may have given her (or a more accomplished actress), but — despite her parents having been killed and her home destroyed in the Blitz — her inner turmoil is left unexplored.

Dawsey Adams, the apparent shy, gruff farmer of the novel — as played by Michiel Huisman — is missing entirely from the film. Huisman may have the rugged good looks required but lacks the essential vulnerability. Juliet’s bonding moments with Dawsey are pure cinematic convention rather than kindred spirit.

Fortunately, this lack is more than made up for by a number of finely measured supporting performances. Penelope Wilton (Downton Abbey, The Best Exotic Marigold Hotel) as the matriarchal Amelia, is tightly wound and deeply wounded by the events of the war. Wilton — apart from the glorious Guernsey landscape — is the star of the film. She simmers with resentment and grief while having to present a cheerful face.

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Lily James is surrounded by a fantastic supporting cast on the island of Guernsey

It is Katherine Parkinson (The IT Crowd, Humans) as Isola Pribby — the spinsterish gin lover— who, surprisingly, gives the film much humour and warmth. Initially appearing trite, she develops into a wise, true friend for Juliet, and it is heartening to see Isola’s loneliness diminish with Juliet’s company.

Of course, it is Elizabeth (Jessica Brown Findlay — Downton Abbey, This Beautiful Fantastic) who is the connective tissue of the story — mysterious, beloved, and missing — which allows Juliet to adopt the role of detective, to unpick what happened to Elizabeth during the war, and discover why the society are so reticent to speak of her.

Despite Elizabeth appearing only in the few tantalising flashbacks — when Guernsey was occupied by German forces — Findlay is assured in the role, having moved on in leaps and bounds from her restricted character in Downton Abbey. Her Elizabeth is fiercely independent, compassionate, brave, and sympathetic.

Indeed, the entire supporting cast are superb, and steady the picture. Tom Courtenay (Last Orders, The Golden Compass) as postmaster Eben Ramsay (creator of the Potato Peel Pie) is wonderfully wry, and Kit Connor (The Mercy, Ready Player One) is precocious as his son Eli. Though Matthew Goode (Brideshead Revisited, A Single Man) as Juliet’s publisher, Sidney, is beginning to wear out his standard stiff-lipped Englishman.

Mike Newell (Four Weddings and a Funeral, Harry Potter and the Goblet of Fire) directs leanly and makes sensible decisions to show flashbacks only when necessary. He is an unsentimental director, and in moments of great emotion, he trusts his actors to play true — a decision which heightens the drama, especially in the deeply affecting climactic scene.

The screenplay is acceptably functional. Though much is always lost in adaptation, it is a pity not to have delved into the German occupation more. Held off as it is by the characters’ stories, it becomes mere anecdote, which may be the point but renders them less real. It is also a pity that we do not discover the society through their own letters, which was such a delight in the novel.

Zac Nicholson’s (The Honourable Woman, The Death of Stalin) cinematography is lush. I expect Guernsey’s tourist trade will soar — despite Devon and Cornwall standing in as locations for the island. London — emerging from austerity after war — is also evocative: alternately grey and rubble-strewn or garish with cocktails and dancing in lurid nightclubs.

Juliet neatly juxtaposes the abrupt excess of celebration after the privations of war — “I feel like we’ve emerged from a tunnel into a carnival.” Yet, as she discovers when she journeys to Guernsey, the war is still close and emotions run hot.

The Guernsey Literary and Potato Peel Pie Society is a faithful, pleasant adaptation that fails to fully capture the joyous spirit of the novel. But its central conceit — the power of books to transform grey worlds, to bridge hurt; books as refuge, books as intimate connections to new friendships and to new love — still charms enormously.

Have you read the book? What did you think of the film? What are your favourite film adaptations of novels?

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