Roy Keane spent five months as assistant manager to Paul Lambert at Aston Villa in 2014.
Keane combined the role with also being part of Martin O'Neill's Ireland management team. He left Aston Villa in late November that year citing difficulties with combining both jobs.
Speaking Talksport on Monday, Darren Bent - who was at Aston Villa at the same time as Keane - recalled working under the Corkman.
"He was really, really scary," said Bent.
He would just lose it. I can understand, when you've been a top player like Roy Keane was, one of the very best, in his head he can't understand why us players at Aston Villa make certain mistakes. It just doesn't compute with him, 'How can you not pass from there to there?' He just can't see it.
After a 2-0 defeat to QPR at Loftus Road in October 2014, Bent said Keane castigated the Villa players for two hours in the dressing room.
"Paul Lambert stepped to the side and Roy just started talking," said Bent.
"It was a night game. I promise you, all the lights in the building [had been turned off], everywhere; all the families waiting outside had gone because we'd been in the changing room for two hours. He just went off for two hours.
"It went from talking about the Lucozade drinks that people drink, players getting too many massages, people doing yoga.
"Ron Vlaar had a knee or ankle injury, or shin splints, and he was just like, 'How can a player be out for that long with shin splints?'
"He just went scanning around the room picking faults. No one wanted to make eye contact with him because the moment you did, he was going for you as well.
"Everyone just sat there for two hours. You went outside, all the lights had gone off, the families had left.
"I'm still traumatised by what happened that night. Honestly, he was crazy."
😰 “He was really, really scary! He would just lose it!”
🤬 “After a game Roy just went off at us for two hours!”
😤 “He talked about drinking Lucozade, getting too many massages, yoga...”
😳 “He was crazy!”@DarrenBent reveals what life was like under Roy Keane at #AVFC! 👀 pic.twitter.com/BaSGikLFqS
— talkSPORT (@talkSPORT) June 22, 2020