A boy told garda interviewers he ran away when he realised his friend was raping schoolgirl Ana Kriegel, a trial heard.
Boy B also said he saw Ana and Boy A "struggling" before she was "flipped" on to the ground.
Boy B said Boy A then climbed on top of 14-year-old Ana, started choking her and "slowly" taking off her clothes.
Boy B told gardai he then realised that Boy A was "raping" Ana and he "had to run".
Boy A had a "blank face" and was showing no emotions while Ana was crying and begging him to stop, Boy B said.
The two accused, who were both aged 13 at the time, have pleaded not guilty before the Central Criminal Court to the murder of Ana Kriegel at Glenwood House, Laraghcon, Clonee Road in Lucan on May 14 last year.
One of the boys, Boy A, has also denied a charge of aggravated sexual assault.
Yesterday the jury continued to watch video footage of interviews which gardai conducted with Boy B after he was arrested for a second time on suspicion of the murder of Ana.
The interviews took place on July 7, 2018.
Present during interviews six and seven were Boy B, his mother, his solicitor Barry Powderly and Detective Garda Donal Daly and Detective Sergeant (now Inspector) Damien Gannon.
Gardai told Boy B that they had new information and new evidence and the questions they would be asking him were relevant to that.
In the interview, Gda Daly put it to Boy B he had told "numerous versions" and the only version he wanted was the truth.
"Just the truth [Boy B]," Gda Daly said. Boy B said he walked into the abandoned house, looked around, picked up a stick or white plank and then put it down.
He walked back outside and Ana and Boy A walked into the house. Boy B said Boy A then told him to leave.
Boy B said he stood by the doorway and watched as Boy A "took her down and stripped her". Boy B said he then ran out of the house.
Asked to describe exactly what he saw, Boy B said he saw Boy A on top of Ana, taking her clothes off.
He said he saw Boy A's construction boots, black bottoms and black hoodie.
Boy B said he could see Ana crying.
Boy B said he guessed that Boy A's backpack was in the corner.
In his interview, Boy B said he was standing there, staring, wondering what he should do. He could see Boy A choking Ana.
Boy B said Boy A and Ana were both on the ground "struggling".
Asked about Boy A's hoodie, Boy B said it was up and he couldn't really see Boy A's face.
Boy B said his brain realised what was happening. Boy A then turned his face and he had "this look on his face".
Boy B said he couldn't describe the look.
He was then shown a photograph of a stick which was found near Ana's body, but he told detectives this was not the stick he had picked up as the one he picked up did not have red stains on it.
Asked if he had touched the stick in the photograph, Boy B replied "No".
Boy B also described the condition of the rooms in the abandoned house, saying there was a lot of trash, rubbish and sticks lying around.
Asked to describe Room 1, which is the room where Ana's body was later found, Boy B said he saw beer cans, rubbish, rocks, sticks, graffiti and planks.
During the seventh interview, Gda Daly asked Boy B to give him a guided tour of what he had seen.
Boy B said he walked out of the house and Ana and Boy A were still talking.
He walked up to them and they started walking towards the house.
Boy B said he went to follow them but Boy A said to him "Hey [Boy B] can you leave please".
Boy B told gardai he did not really want to leave. He thought something was going to happen in terms of Boy A and Ana.
He said he thought Boy A was going to kiss Ana and she was going to kiss Boy A and they were going to "ask each other out or something".
Boy B said he went into another room but he heard "shuffling" from the room Boy A and Ana had gone into.
Boy B said he went over to see what was happening. He stood at the door and saw Ana and Boy A.
At first it looked like they were hugging. He couldn't see their faces. Both were struggling and then Boy A flipped her on to the ground.
Boy B said Boy A climbed on top of Ana and started choking her with his right hand. He slowly took off her jumper, T-shirt and her pants.
Boy B said that Boy A threw Ana's pants to the side.
Boy B said he then realised Boy A was raping Ana and he "had to run".
Just as Boy A was about to take off Ana's bra, Boy B said Boy A started choking Ana again.
Boy B said Boy A then looked up and that's when he [Boy B] ran. He said that he saw Boy A's face and it was a "blank face" with "no emotions".
Ana was crying and kept begging "Please don't do this" and asking Boy A to stop and not to hurt her, Boy B said.
Asked if he took off any of Ana's clothes, Boy B said "no".
Asked if he went into the room, Boy B said "I didn't".
Boy B said the only time he touched Ana was when he gave her his hand earlier when they were crossing a ditch on their way to the house.
Boy B also said he did not see Ana with any injuries nor did she say anything to him when she was lying on the floor.
The trial continues before Mr Justice Paul McDermott and a jury of eight men and four women.