Funny that Brian was recommend by Bon in a way. Bon was aware of Brian from his first band Geordie.
From Wikipedia:
"Following the death of AC/DC frontman Bon Scott on 19 February 1980, Johnson was asked to audition for the band in London. AC/DC lead guitarist and co-founder Angus Young recalled: "I remember Bon playing me Little Richard, and then telling me the story of when he saw Brian singing [with Geordie]." He says about that night: "There's this guy up there screaming at the top of his lungs and then the next thing you know he hits the deck. He's on the floor, rolling around and screaming. I thought it was great, and then to top it off you couldn't get a better encore they came in and wheeled the guy off!'" Johnson was diagnosed with appendicitis later that night, which was the cause of his writhing around on stage.[2] The band agreed immediately that Johnson's performing style fit AC/DC's music."
From what I understand Bon was such a mess at the end, his 33 year old body was really the body of a 60 year old. Bon contemplated one more album with AC/DC then calling it quits, settle down and try to get healthier.