The trailers for Where the Wild Things Are that have been playing for the last six months were so thrilling and so moving — I know adults who reduce themselves to tears watching them on the internet — that it would be almost impossible for any full-length feature to live up to their promise. And the film does not.

That said, Spike Jonze’s long-awaited film adaptation of Maurice Sendak’s classic 1963 children’s book is by no means a catastrophe or a travesty, like the efforts to put the Dr Seuss books on the big screen. Indeed, it is a dazzling achievement in many ways, brilliantly inventive in its efforts to capture the visual and emotional world of Sendak’s 18 pages, and often very beautiful.

Unfortunately, it may be too complicated and too sad for small children and too slow and self-consciously whimsical for adults. Certainly it loses momentum after the second or third time you see the superbly created Wild Things bouncing and bounding around their island. And it often seems to be pulling in different directions, as if the filmmakers could not agree if their story was really about a child’s imaginative world or an expression of young adult cynicism about the adult world.

Of course, to turn an 18-page picture book with only 10 lines of text into a film, Jonze, whose previous work includes Adaptation and Being John Malkovich, and his co-screenwriter, Dave Eggers, had to invent a great deal. In particular they fill out the character of the little boy Max and have given the monsters names, personalities and dialogue. One of them even becomes Max’s special friend. They also invent a backstory. Max (Max Records) here is a child of divorce, living with his mother (Catherine Keener) and older sister but missing his father. He enters the magic world of the Wild Things not from his bed, as in the book, but after running away from home, following a row he provokes with his mother.

He sprints down the street after biting his mother on the shoulder — to the shock of her new boyfriend (Mark Ruffalo) — and finds his way to a sailboat on a lake. Still wearing his towelling wolf-suit, he takes the boat out onto open water and is caught in a genuinely scary storm before landing on an island.

At times the filmmakers try to turn the story into a lesson about running a family

Max is not a particularly sympathetic nine-year-old: he is selfish, sullen, aggressive and desperately in need of discipline. The creatures he discovers on the island are melancholy hybrids of child and grown-up. Rightly, they are scary as well as cuddly, with intimidating teeth and claws, but they do not seem especially wild. When they make Max their king, rather than eating him, it is because they want someone to protect them from sadness. When they are not obeying Max’s orders to sleep in a big pile, fight with clods of earth, or to build a (stunningly envisioned) fort, they are sombre and neurotic.

Visually, however, they are perfect. The huge shaggy costumes for the Wild Things were created by the Jim Henson Creature Shop, responsible for the Muppets, and they are just right in the way that they simultaneously resemble real animals, stuffed toys and things from a nightmare. They are all the more effective because Jonze has used computer-generated animation to make their big, wide mouths move as they speak.

When they do talk it is with the voices of a superb cast headed by James Gandolfini of The Sopranos and supported by Lauren Ambrose, Chris Cooper, Forest Whitaker, Paul Dano and Catherine O’Hara.

Though it sometimes seems that the Wild Things are all supposed to be projections of Max’s own imagination, Jonze and Eggers have chosen to make them sound like middle-aged adults. Sendak has admitted modelling his creatures on his own, less glamorous Jewish relatives. Jonze’s and Eggers’s versions kvetch, kibbitz and argue as if they were residents in a New York old people’s home. At other times they squabble like children.

There is Ira and Judith (Whitaker and O’Hara) as a comfortable but whiny couple; KW (Ambrose), an emotionally mature girl-beast who has brought trouble to the gang by leaving it for new friends at the beach; wise bird-like Douglas (Cooper) and, most important, Carol (Gandolfini), who seems as violently temperamental and vulnerable as Max himself.

Things have clearly gone wrong in their gang in the past, grown-up things like failed love affairs. But it is in the peevish, elliptical conversations they have that the film eventually falters. At times the filmmakers even seem to be trying to turn the story into a lesson for Max about the difficulty of running a family, or even a half-baked political allegory about authority and democracy. When Max finally goes home — the story having all but petered out — it is not “to be where someone loved him best of all” but because his kingship has failed.

Ultimately, it is hard not to wish that the characters would shut up and let the dreamlike images, including the beautifully filmed Australian landscapes, do the talking.