Note that there are possible spoilers in the review, but I haven't mentioned anything that isn't in the trailer.
Despite the obviously dark subject matter Room is not a dark film, it's not one that draws heavily upon the suffering of the hostages and neither is it one that is exploitive as it's told from the child's perspective meaning that the film has a lighter tone than one would originally expect. The trailers did, to an extent, miss-sell the film, selling it as more of a thriller than the profoundly moving and incredibly engaging film that it is. Room is very much a film that is powered by the wonderfully written mother-son relationship that really helps the film become as emotionally devastating and powerful as it is. This mother-son bond is so powerful because of the two exceptional performances by Brie Larson and Jacob Tremblay, playing mother and son respectively.
The two central performances are truly exceptional as they help make the mother-son relationship believable and overwhelmingly emotional. The relationship isn't always plain selling, both have their melt downs and snap at one another but the genuine affection displayed by the pair makes for a heartbreaking relationship. The room means something very different to Jack than it does to the mother. The mother, to protect Jack, has told him that the Room is the entire world, and as the story is told from the child's perspective we get the impression that the room is far bigger than it is (this is owed to the perfect cinematography).
When the pair are in the single room their relationship becomes the central point of the film but once Jack eventually escapes, the child like wonder to what is to him a completely alien world contains a performance of depth and complexity that is far beyond what is expected of someone of such a young age. Jack's tentative first steps in this alien world is just one of many powerful moments, and it's also worth noting that nothing is lost once they pair escape the confines of the room.
The heartbreaking mother-son relationship gives the film everything it needs – tension and pathos. It's a stunning film.