Inside Man's biggest twist was Louis Oliver's show-stealing performance

Harry and Mary's son Ben – the character who in many ways was the catalyst for the drama – was played by Louis Oliver, the son of Inside Man writer/creator Steven Moffat and series producer Sue Vertue. It'd be easy to cry nepotism at this casting choice, though at a BFI screening of the first episode, Moffat insisted that neither he nor Vertue had pushed for Oliver to be cast and that he was treated the same as any other actor, being made to try out for the part of Ben alongside several others.

"We made him audition," Moffat said. "And other people auditioned too and Sue and I were not involved in the casting choice at all, we told Paul [McGuigan, director] that we would forgive him – 'It's fine, go and choose one of these – that one's our son - but it's okay whichever one.'

"And Louis was fine with it, he was saying, 'That's absolutely fine, I’ve got no problem, I understand.' Sue and I didn't sleep for three nights. Three nights worrying about it! And thankfully he got it."

In fact, Moffat revealed that while he'd partly based the character of Ben on Oliver – one interaction between Ben and Harry in episode 1 of Inside Man, where the former tries to convince the latter to pay him to complete his homework, was apparently based on a genuine conversation between Moffat and his son – he hadn't actually considered casting him until a recommendation by Mike Flanagan, who had previously hired Oliver for his Netflix series Midnight Mass.

"He was recommended to me, genuinely, by Mike Flanagan," Moffat said. "He did a show you must watch on Netflix, Midnight Mass, which is fantastic and Louis was in it, and Mike Flanagan sent me an email saying, 'He's brilliant, your son.' And we were looking to cast a Ben and we thought 'Well, OK, Ben’s a bit based on Louis,' and we just thought maybe we should try."

Still, it'd be easy to remain cynical – except for one thing. The fourth and final episode of Inside Man delivered perhaps the show's most unexpected twist yet, as a top-flight cast including the likes of Tennant, Wells, Marshal, BAFTA-nominated Lydia West and Oscar-nominated Stanley Tucci all found themselves overshadowed by Oliver, who delivered an absolute tour-de-force performance.

The earlier episodes of the series, it's fair to say, hadn't done much to really test Oliver or showcase his abilities – with Ben out of the loop with regards his parents' nefarious activities, the character wasn't required to do much beyond typical moody teenager conduct, bar the odd suspicious look shot at the vicar and his wife.

The final episode, though, saw Ben finally discover the horrifying truth. What's more, he found himself locked in the same cellar where Harry had been holding Janice hostage and subject to the same fate that Harry had planned for his captive – a slow, unpleasant demise from carbon monoxide poisoning (via a broken gas heater).

Ben gradually lost his wits, growing increasingly unhinged and desperate as the full reality of the situation dawned on him at the same time as the poisonous gas began to impede his ability to think straight. This wild-eyed, anguished, almost rabid Ben was a completely transformed figure – and Oliver absolutely delivered in these pivotal scenes.

Tennant's Harry underwent a similar switch when intrepid journalist Beth Davenport (Lydia West) entered the cellar, shedding his well-cultivated public image as the friendly, approachable vicar to reveal the animal beneath – and it's testament to Oliver's work in Inside Man that Ben's meltdown was every bit as convincing as that of his on-screen father, played by an actor with dozens of award wins to his name.

Prior to watching Inside Man, it's understandable that some might seek to question Oliver's hiring, but after watching, it's difficult to imagine anyone questioning his talent. Whether one believes he had a helping hand or not is almost immaterial when the end result more than justifies his casting. Ultimately, it's clear the young actor has a bright future ahead of him, whether his parents are involved in his career or not.

Read more:

• Inside Man ending explained: How did the twisted web unfurl?
• Steven Moffat says Inside Man is "self-contained", won't rule out sequel
• Inside Man soundtrack: What is the theme song for Steven Moffat's drama?
• David Tennant almost missed out on Inside Man role

Inside Man is available to watch in full now on BBC iPlayer. Check out more of our Drama coverage or visit our TV Guide to see what's on tonight.

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