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The teenager - whose identity is suppressed - is being grilled by defence counsel Jonathan Eaton QC who suggested he must have played more of a role in the death of 16-year-old Amber-Rose Rush than he had disclosed.
His client, Dunedin doctor Venod Skantha (32), is on trial in the High Court at Dunedin accused of murder, as well as four counts of threatening to kill.
He allegedly stabbed Amber-Rose in the neck six times and left her to die in her bed at her Corstorphine home.
The teenager, who allegedly drove Skantha to and from the Clermiston Ave address, said by the time of her death on February 2 last year, they were not as close as they had been.
At one point, the witness accepted, he had told Amber-Rose he loved her.
Mr Eaton asked if he was ''a little bit obsessed''.
''Oh no,'' the teen said.
Mr Eaton raised an incident when Amber-Rose allegedly found him standing over her in the darkness of her room.
The witness argued he was not ''creeping her out'' and her mother had let him in that night.
Skantha's lawyer immediately set out the way he saw the case.
''The defence position is that you have way too much detailed information to have simply got from Dr Skantha and the only way you could've had that much of the detail is if you were involved personally,'' Mr Eaton said.
''And the only reason you've been so inconsistent in the accounts you've given is because you're protecting your own position to implicate Dr Skantha.''
The witness denied that was the case.
In his interviews with police during February, the teenager said he was a ''compulsive liar'' but yesterday he sought to clarify that.
''What I meant was I tell white lies. I didn't mean to say 'compulsive','' he told the court.
Mr Eaton pointed out the witness had told friends Skantha had threatened him with a knife at one stage.
He admitted he had been exaggerating.
Six days after Amber-Rose's death, the teen contacted her mother, Lisa-Ann, and gave her that version.
''I just want to say when that scum got me to drive to your house, I had no idea what was about to happen ... [he] pointed a knife at me and threatened to kill me and my family if I didn't drive to his house,'' he wrote.
The witness said he enjoyed the attention the case had brought him.
He was shown messages he exchanged with more than one media outlet.
''I know stuff about doctor charged with murder,'' he messaged one.
Another asked whether he would speak exclusively on camera but police intervened before it went ahead.
The teen asked to be paid for the interview, the court heard.
''The reason I went to the media was because it was attention,'' he said.
Skantha and Amber-Rose had a heated exchange on Facebook Messenger, which the Crown said directly resulted in her murder just minutes later.
The defendant had been alerted by his teenage friend to a screenshot of the private conversation the victim had posted on Instagram.
''It really upset you,'' Mr Eaton suggested to the witness.
He denied that was the case and said he was not seriously concerned Skantha would lose his job.
In his interviews with police, the teenager said he was instructed by the defendant to clean blood off his shoes but he deliberately neglected the task so the man would be identifiable as the killer.
''It sounds like a good script for a film. A 16-year-old boy sets this trap for a 30-year-old doctor and solves the crime,'' Mr Eaton said.
''I don't know what to say to that,'' the teen replied.
The cross-examination will continue today.