It starts with a baby shitting in your mouth and only gets worse from there.

The central conceit of The Change-Up is that urinating into a fountain can turn you into Ryan Reynolds. That being the case, I should think the entire audience would leave early just to pave over their water features. Being Ryan Reynolds might have its upsides – Olivia Wilde, in this instance – but the payoff is Jason Bateman systematically destroying your life. This, from a man we once considered a moral stalwart amongst thieves, philanderers and Bluth never-nudes. Just how badly could the most put-upon man in cinema really screw things up, you might ask?

At least to begin with, not very. While Mitch (Reynolds) structures his days around several key responsibilities, namely getting baked, getting laid, and getting baked after getting laid, Dave is studious to a fault. On the fast track at a prestigious law firm, he’s so focused on his career that he neglects his wife and two young children; you see now where the shitting comes in. As they must, Mitch and Dave drunkenly envy one another’s lives, and a timely lightning bolt duly grants them their wish. Ridiculous? Sure, except Big once conceived of a broken Zoltar machine that could make a grown woman fall for a small boy pretending to be Tom Hanks, and how fucked up was that?

While the old Dave adjusts to his newfound bachelorhood, the formally-known-as-Mitch takes care of the kids. Ordinarily, that most creaking of movie cliches would be set to work exploring the banal truisms of family life, but in a summer of R-Rated comedies, such a thing will not suffice. Instead, we indulge an ill-functioning manchild who behaves with such petulance that it’s unthinkable he would ever have found a friend to swap with in the first place. He’s a psychopath with none of the positive attributes, while his babysitting of two children who throw knives and embed themselves in plug sockets is straight from a Child’s Play / Final Destination crossover. With very little imagination, the film’s tagline could easily have read ‘Defecation, lactation, titillation and masturbation’.

For a good while, the movie seems beyond saving. Then something quite startling occurs: Leslie Mann emerges as a point of real sympathy. Crucially, she does so not by lamenting her husband’s new found narcissism, but in revealing it had always been there. The Change-Up is transformed the second it foregoes mindless dick jokes in favour of the deeper truths to these characters. The jokes that remain are kinder somehow, arising mainly from children’s expectations of their parents, and how that might lead a bullied 9-year-old to fixing their problems with a rusty shank.

Kindness is relative, of course.

Mann and the kids are reliably endearing, even if Bateman plays from a deck of sadism so spiteful you can’t quite believe the film got made. Or you can, and that disturbs you even more. The entire first hour is demonstrative of a film that has it all with plenty to spare, and still comes up wanting. Playing against type might work as a vague concept, but the very process of inhabiting other characters leads to actors substituting their charm for something foreign and contrived, and therein lies the failing of all body-swap comedies. Manage to look beyond that, and an infinitely sweeter film awaits. I didn’t even mind that everyone learns what’s really important in the end; only that it takes so long to get there.