Having previously teamed up for 2011’s Bad Teacher, director Jake Kasdan reunites with Cameron Diaz and Jason Segel for R-rated comedy Sex Tape. Though it starts strong and finishes well, script flaws mean it’s unfunny for the majority of its run time, squandering the potential of a great cast and an entertaining premise.

Segel and Diaz play Jay and Annie, a married couple whose sex life has dulled in the ten years they’ve been together. With the kids out of the house, the two eventually decide to rekindle their intimacy by making a video of themselves trying out every position in ‘The Joy of Sex’. Though the gambit works, Annie and Jay soon discover that their private video has gone public.

That’s a perfectly fine setup, and the opening act is a strong one too. Crucially the film doesn’t begin, as so many others in the genre do, with the central couple hating each other. Indeed, their genuine like for one another is evident and refreshing, and early scenes feature honest, efficient observations on how life’s demands can affect a couple’s closeness.

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It’s once the narrative comes into focus that the missteps in the screenplay co-written by Kate Angelo, Segel, and long-time collaborator Nicholas Stoller arise. The manner in which Jay and Annie’s sex tape is made public relies on a somewhat laboured plot device which sees the video being synced to the iPad’s of friends, family and colleagues. Worse than that though is where they go from there; rather than having Jay & Annie deal with the fallout from having their friends watching their most private moments, instead they try and hunt all the iPad’s down before any such scandal occurs. It sounds humorous in theory, but in practice it’s anything but, and not even Segel’s occasionally impressive improvisational skills – which feature prominently in an admittedly funny sequence with Rob Lowe’s volatile businessman – can permanently fill the largely laughless void.

If Sex Tape is short on belly laughs then it’s even skinter when it comes to sharp insights. With Jay & Annie hesitant to confront the issue head on, the film gets further and further away from the message it’s trying to perpetrate. By the time any form of real perceptiveness comes – delivered by an unbilled star whose cameo we won’t spoil – it’s too little, too late.

For their part, Segel and Diaz have solid chemistry with one another, which makes it all the more frustrating that the script doesn’t develop their characters as much as it ought to. Similar can be said of Rob Corddry and Ellie Kemper; playing a couple who are friends with Jay and Annie that develop a keen interest with the sex tape – incidentally meaning that this B plot is far better than the A plot – they play off one another well and score some laughs, but you don’t get the sense that their relationship has deepened once the credits roll.

It’s unfortunate then that a game cast and a couple chuckle-worthy sequences do not a satisfying comedy make. Neither raunchy nor insightful enough, this Sex Tape underwhelms.