This is the chilling moment a woman laughs like a "giggly schoolgirl" as she lies to police after helping to kill her partner's step-sister.
Shauna Hoare and boyfriend Nathan Matthews were jailed for a total of 50 years over the death of 16-year-old Becky Watts in a sexually-motivated kidnap plot.
Matthews, who suffocated his stepsister in February 2015, was found guilty of her murder, while Hoare was acquitted of the charge but convicted of her manslaughter.
A new documentary has revealed how Hoare smirked and smiled during interviews with police following the disappearance of the teenager.
The pair had originally pleaded their innocence to officers.
The footage shows how Hoare "seems like a giggly schoolgirl" and is trying " to give the impression of someone who is helpful", experts said.
In the documentary, experts in pyschology, body language and speech analyse six taped interviews with police officers and pinpoint the moments where Matthews and Hoare give away their guilt to police.
When asked to say her full name to police, Hoare seems to sit up straight, smile and announce her full name of Shauna Stacey Charmaine Hoare.
Experts say she is trying to appear like she "doesn't quite understand the seriousness" of what is going on.
Body language expert Cliff Lansley said in trying to convince the interviewer, Hoare "overeggs" her statement and her shoulder movements contradict her words.
Hoare is currently serving a 17-year sentence for manslaughter.
This is while Matthews is serving at least 33 years in prison.
