One of the great things about American films is that they can take serious subjects and turn them into popcorn movies.

Argo did it last year, finding humour in the 1979 Iranian hostage crisis. Two films competing with Argo for the Oscar next month – each hugely entertaining – deal with American slavery. However, Steven Spielberg and Quentin Tarantino have hugely contrasting approaches. Spielberg's Lincoln is to be released next week. For now, Django Unchained, while hardly the seminal commentary on one of America's most shameful episodes that Tarantino claims, is a bold and at times exhilarating satire that makes an intriguing counterpoint to Spielberg's respectful biopic.

Having made a decent fist of the war movie genre with Inglourious Basterds, the writer-director turns his attention to the western. As with the previous film, he honours the genre while adding his own distinctive embellishments, not least some perverse revenge-empowerment. In the earlier film, it was a band of American Jews with a mission to scalp Nazis. Here a freed slave-turned-bounty hunter who, when asked if he likes his new profession, answers: "Kill white people and get paid for it – what's not to like?"

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The opening scene establishes the template – lots of talk, followed by a flurry of comic-book violence. Under the guise of a harmless dentist, experienced bounty hunter Dr King Schultz (Christoph Waltz) sweet talks two men leading a group of slaves through the desert, before despatching one, allowing the slaves to deal with the other, and hiring Django (Jamie Foxx) as his new sidekick. The black man and the European form an immediately effective alliance, whose novelty is best summed up by the shocked language of the locals the minute they enter town.

Now the "n" word has got Tarantino into trouble before, but it's perfectly justified here – this is an environment where it will have been commonly used – even if the director does relish its repetition. His way of dealing with such a sore subject as slavery is to send it up, the most uproarious and daring example being a scene in which the Klu Klux Klan descend on our heroes, only to be undone by an inability to see through the holes in their new hoods. Meanwhile, Tarantino allows his slaves a response to their oppressors that they would not have had in reality. As Django seeks to free his wife and fellow slave (Kerry Washington) from psychotic plantation owner Calvin Candie (Leonardo DiCaprio), the body count soars.

And so the film is undone by Tarantino's customary flaw: self-indulgence. The dialogue is fabulously smart and funny, the performances, especially that of Waltz, superb and the vibrant love of cinema is evident. But, once the originality has worn off, and we realise Tarantino has nothing interesting to say about slavery (despite the endless talking), the brutality and blood become tiresome. If he made movies for others, not just himself, he'd be creating unimpeachable classics rather than enormously creative films that leave one wishing for better.

At least Tarantino's violent tale has cinematic panache. Ray Burdis's The Wee Man merely has violence and a flimsy moral message behind its rendering of the life and times of reformed Glasgow criminal Paul Ferris. There has been much local controversy over and obstruction to this film being made. Nevertheless, the result may have been interesting had it been tougher. Instead, there's a sense of hagiography about it, an attempt to explain a villain's life through his environment, which ignores the fact that many young men survive bullying and gang culture without turning bad themselves.

The absence of any truly illuminating psychology undermines Burdis's script. And so, as we watch Ferris (Martin Compston) rise through the ranks of the Glasgow underworld in the 1970s and 1980s – beating, intimidating, murdering – it's impossible to sympathise or engage with him or anyone in his orbit.

The Sessions is also based on a true story, although an altogether more life-affirming one. Mark O'Brien was an American who was paralysed from the neck down after contracting polio as a child, yet succeeded as a poet and journalist, leaning on his wit and Catholic faith to get him through frequent despair.

The film focuses on a few months in 1988 when an assignment to write an article about "sex and the disabled" stirs the thirtysomething virgin (John Hawkes) to seek the seemingly impossible. Having received the blessing of his enlightened priest (a wonderfully droll William H Macy) he hires sex surrogate Cheryl (Helen Hunt) to help him discover sex. The story is structured around their sessions, and the complicated emotional bond that develops from this unusual intimacy.