Starring Julia Roberts, Lily Collins, Armie Hammer • Directed by Tarsem Singh • Age restriction: 10M • Releases 20 April

It's always frustrating to watch a movie with a great idea but lacklustre execution. It's even worse when there are moments of brilliance that are smothered by the rest of the film's mediocrity. Mirror Mirror is one of those movies.

Directed by Tarsem Singh, Mirror Mirror is a typical Tarsem Singh movie. Like The Cell and Immortals, this adaptation of the Snow White tale is heavy on the visual appeal, but lacking in substance and depth.

Lily Collins stars – but you'd never think it, because her performance is so bland – as Snow White, a beautiful and kind-hearted girl that has been locked away in the castle tower after her father's sudden death at the ends of the Beast in the forest.

Meanwhile, the Queen (played deliciously by Julia Roberts) has bankrupted the kingdom with her extravagances – that include lavish parties, enormous gowns and a gigantic golden throne – and must now marry somebody rich to keep up her lifestyle. When the handsome, powerful and ridiculously rich Prince Alcott (The Social Network's Armie Hammer) is robbed and must take refuge in her castle, the Queen marks him as her meal ticket – and pulls out all the stops to impress him. Unsurprisingly, the Prince has already fallen in love with Snow – who he met in unusual circumstances in the woods – and after realising that she has competition, the Queen decides she wants Snow dead and sends her into the forest to be murdered. Instead, the Queen's servant (Nathan Lane) lets Snow go – and she moves in with the Seven Dwarves (a group of seven highway robbers in this adaptation) and plots to overthrow the Queen.

Thank goodness for Julia Roberts. She steals the show as the Queen, gamely camping it up and bringing an aging gold-digger spin on the traditional Snow White villainess. Her mirror too is a reflection of herself – although, as the mirror-Queen says, not a true reflection as she doesn't have wrinkles. "They're not wrinkles!" the Queen moans. "They're crinkles!" Roberts is wild and absorbing, doing her best to spruce up the often tired and cheesy jokes written for her. She's given the inverse of her famed rom-com roles: Try as she might, she just can't seem to land the prince... Until she gives him a dose of love potion, and even that doesn't work out that well.

Lily Collins, on the other hand, is positively invisible. Beautiful, yes, but little more – and she pretty much blends into the background. Other than being pretty and nice, there's no real reason to like her or even cheer her on, and even when she undergoes under a dwarf-supervised makeover from wilting flower to sort-of-kickass warrior princess, it's still not enough. Her performance – and the character she is given – is uninspired, and try as I might I couldn't think of her as anything other than a "nice girl".

There are great performances from some of the supporting characters – among the dwarves, Mark Povinelli as Half Pint, Danny Woodburn as Grimm and Martin Kebbler as Butcher are the  stand-outs – and Nathan Lane provides ample nervous comedy and a slightly uncomfortable joke hinting at sexual assault at the hands of a grasshopper. Finally, Sean Bean brings some much-needed gravitas in his small role as the ill-fated King and Snow White's father.

Finally, Armie Hammer, who won praise for his dual role as the Winkelvoss twins in The Social Network, has been given more than Collins in the script to play with – but standing tall at 6 ft 5, he often comes across as awkward and bumbling in sets that seem far too small for him. Like Roberts, Hammer is perfectly content to have fun with his role – and though he never achieves any form of brilliance, he is at least enjoyable to watch. He's given a few potential ultra-humiliating moments that he tackles with grace and great humour – licking Roberts' face when the Prince is accidentally given "Puppy Love" potion, the two can barely contain  their grins ... And it's good to know that at least the actors seem to be having fun.

There are some wonderful moments of self-awareness too that spark up the film, but these are easily counted on one hand. For example, when Snow charges out into the woods to try to single-handedly save the day, the Prince shouts after her not to mess with traditional storytelling: "It's been focus-tested!" he shouts. "We know that it works!"

And this is the crux of Mirror Mirror's failure – messing with the story, trying to make it different and original, but not really committing and going all the way. There are the dwarves – now highway robbers, with a surprisingly tragic back-story – who become, as usual, comic relief. The Prince points out a tired version of the "It's funny 'cause they're short" idea. Also included in the film is probably one of the most awkward and uncomfortable song-and-dance moments I've ever seen. (Seriously. I had to keep averting my eyes). Collins - daughter of legendary musician Phil Collins - sings a track called I Believe, which is heavy on the autotune and feels so out of place, more on the stage than on the screen. That being said, I'm pretty sure the pre-teens the film seems to be targeting will love it.

In general, Mirror Mirror has a holiday-pantomime feel. The sets feel small, cramped onto tiny stages, and pretty much all of the film's props look as though they're made of plastic. Most fun are the costumes, designed by the late Eiko Ishioka – who won an Academy Award for her work on 1992's Bram Stoker's Dracula. There are massive, richly-coloured gowns, dripping with ostentation; lavish menswear and wonderful fighting gear. During the ball the Queen throws the Prince, each guest is dressed to resemble an animal – and Snow dons an enormous white gown and a head and neck as headgear; while the Prince makes repeated jokes about his bunny outfit. Laughable though they may be at times, the costumes are deliciously over-the-top and delightful fun.

But that's pretty much where the fun stops. There are some fun jokes, some moments of brilliant self-awareness and a fair amount of physical humour – but Mirror Mirror is missing the heart that could have taken it to the next level.

In the veritable plethora of Snow White adaptations, one thing is clear: Mirror Mirror is not the fairest of them all.