A Confession writer honoured victim's family's wishes to not show killings on-screen

Written by Jeff Pope, the six-part series begins when 22-year-old Sian goes missing after a night out in Swindon in 2011. DI Steve Fulcher (Martin Freeman) is determined to find her alive and so, when taxi driver Christopher Halliwell (Joe Absolom) is arrested, he breaks police rules by questioning this suspect for hours without a solicitor.

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Fulcher's approach pays off. But when Halliwell does reveal the truth, he actually leads police to Sian's body – and confesses to her murder.

Liam said: "As soon as we'd seen what Jeff had produced, we were happy with what was produced. It shows Sian as a person, it doesn't focus on Sian's life being taken.

"Which was important, for us. Because we didn't really want that to be a focus, we didn't want to have to envisage that or see it played out. So the fact that it steers away from that and just shows Sian as a person, and then also tackles Steven Fulcher and his actions – it's a good piece of work."

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Played by Florence Howard, Sian appears in the drama as a family-focused, bubbly, positive young woman, deeply in love with boyfriend Kevin Reape (played by Charlie Cooper) and looking forward to having her own children.

Liam recalled: "She was an amazing sister, bubbly, very positive... I don't think anybody who knew Sian would have a bad word to say about her, because she just generally would light up a room."

Having worked closely with the O'Callaghan family from the beginning, Pope said: "The big thing, which was actually Liam's big thing, is he wanted her to be a person, not just a figurehead. So that did affect my thinking... I tried to work Sian in, not just 'that girl that was murdered'."

The family have also thrown their support behind Fulcher, whose career in the police force was destroyed by the Halliwell case.

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Because he didn't caution Halliwell and failed to bring him immediately back to the police station to speak with a solicitor, Fulcher was disciplined for breaching PACE – the Police and Criminal Evidence Act 1984. He ultimately felt he had to resign from the police force.

But the O'Callaghans still believe Fulcher did the right thing.

"He acted as you would want a senior police officer to act if a loved one goes missing," Liam said.

"You want him to try and use any option available to him, within reason, which he did. It's just deemed that in the court of law, his actions were not right...

"What I think this drama will do is highlight both sides – the legal and the moral, with Steven Fulcher's actions, and allow people to have that discussion in their living rooms. And that's quite powerful."

He added: "In some ways watching this drama has made me want to reach out to him again, just to say thank you again."

Sian’s body was found in a shallow grave near Uffington in Oxfordshire, on 24th March 2011, five days after she went missing. She had been stabbed in the head and strangled. Her funeral took place on 18th April.

A Confession airs from Monday 2nd September at 9pm on ITV