WATCH: How subtle body language gave away killer who drugged and smothered fiancée
The subtle body language movements of the man behind the murder of UK author Helen Bailey show how he tried to dodge police questions and cover up his cold-hearted crime, a new documentary reveals.
Ian Stewart, 56, repeatedly drugged his fiancée and smothered her before he dragged her body into a sewer below the £1.5 million ($AU 2.5 million) home they shared in Royston, Hertfordshire in England.
She was found in the cesspit hidden below their garage in July last year, three months after he claimed she had apparently disappeared because she "needed space", The Mirror reports.
Stewart was jailed for at least 34 years and will not be eligible for patrol until he is 90.
As he was quizzed by police about the disappearance of the writer, he appeared harassed and confused.
His lies quickly began to unravel as police continued questioning him.
He had spent weeks poisoning his partner with prescription sedatives, before she one day fell asleep at her computer.
Police allege his motivation was greed, and he was trying to inherit Bailey's £4 million ($AU 6.79 million) fortune.
In Investigation Discovery's Faking It: Tears of a Crime, experts go back and re-examine police footage of Stewart to reveal how his body language was used to help buy himself extra time to answer police questions.
Footage from the investigation shows Stewart looking disengaged, lethargic and bored.
He claimed he was ill and tired, using excuses to avoid certain questions.
In his police interview, Stewart remained silent during five hours of questioning.
It was his uncomfortable movement in his chair, and the fact he put his hand over his mouth and looked disengaged that body experts say was his downfall.
“Putting your hand over your mouth can be a way of making sure you don’t slip up," Professor Dawn Archer, who has analysed Stewart's body language, said.
"Another point at which he also seems to be wanting to cover his mouth, is when they talk about the finances. He’s uncomfortable.
"He looks to all intents and purposes as though he’s disengaged but he puts his hands through his hair, he’s actually concerned, he’s trying to disengage but some of these questions and some of the pauses are actually getting to him.”
It was also when the topic of money came up that Stewart couldn't control his reactions.
“He decided to be silent and the only point he breaks that silence is when the issue of the benefit of money comes up - he has to change his posture, he rubs his arm, he brings his hand to his chin and he speaks for the first time," Cliff Lansley, Body Language Analyst, said.
Police became increasingly suspicious after Stewart jumpily told them "you won't find anything in the garage" when they first asked to search the couple's home.
He had parked his jeep over where police would later find Bailey's body.
Bailey had written 22 titles and was last seen on April 11, 2016.
Stewart was in line to receive two homes, enough cash to ensure a comfortable lifestyle, and pension and life insurance payments from his fiancée.
