In 2009 the witness dubbed Person Four was on his first deployment in Afghanistan as part of the decorated soldier's patrol.
Before the mission to a Taliban compound nicknamed Whiskey 108, located in Uruzgan province, the former SAS corporal is accused of mentioning multiple times the need to "blood the rookie".
The phrase references a junior soldier getting their first kill in action.
According to court documents, Person Four was ordered by his patrol commander to execute an older Afghan prisoner, and allegedly placed him on his knees and shot him in the head.
Mr Roberts-Smith allegedly did nothing to dissuade the execution command, or stop Person Four from following the order, and is complicit in and "responsible for murder," according to the defence file.
But the first Australian witness to give evidence on behalf of the media outlets which seek to rely on a defence of truth, went further in his testimony and alleged it was Mr Roberts-Smith who ordered the execution.
The witness codenamed Person 41 said he was witness to Mr Roberts-Smith and Person Four in a courtyard, standing near the squatting prisoner.
He said he found it "strange" the two soldiers asked to use his suppressor, that Person Four fitted onto his gun.
Mr Roberts-Smith then grabbed the Afghan man by his shirt and kicked his legs out to force him to kneel before Person Four, the witness said.
"RS pointed to the Afghan and said 'shoot him' and stepped to the side," he said of the instruction issued to Person Four.
The witness, not wishing to see "what was about to happen", said he moved into a room and heard a single, suppressed shot from an M4 carbine rifle.
He waited another "15 or so seconds" before re-entering the courtyard, where only Person Four and "a dead Afghan at his feet" remained.
Mr Roberts-Smith vehemently disputes those accounts and is suing The Age, The Sydney Morning Herald and The Canberra Times for defamation over reports that he committed war crimes and murders in Afghanistan between 2006 and 2012.
The 43-year-old - one of a handful of Australian recipients of the Victoria Cross since 1970 - has suggested the execution claims and others stem from jealous associates spiteful of his medallic achievements.
The trial continues.
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