AC/DC's lead guitarist, Angus Young, says he first noticed something was wrong with his brother and founding band member, Malcolm, six years ago when they were recording their album Black Ice.
"When we were writing songs together, me and him, then, it was noticeable," he told ABC's 7.30 program.
"It was just strange things, you know? I mean, memory things."
It was publicly announced in September that the rhythm guitarist and songwriter had been diagnosed with dementia, and had left the band for good.
It was a shock to fans, but not to the band.
"Malcolm was always very organised [so] it was kind of strange. For the first time I'd seen him disorganised, being confused about a lot of things," Angus said.
"That's when it kind of, you know, hit me. Something was not right with him.
"When we were actually doing that album, it became even more prominent and then he would be confused just travelling somewhere and stuff."
Angus spoke to Malcolm, telling him he did not need to keep going if he did not feel up to it.
"And he said 'no I'll keep going, you know, until I can't'," he said.
But eventually, Malcolm just could not remember his own songs anymore.
The band had to make a decision about whether to go on.
"To me it was always up to Angus, really," AC/DC lead singer, Brian Johnson said.
"Because Angus was the part of these two men who made these riffs that nobody else made - for a long time, since they were at school.
"If Angus turned around and said, 'I can't do it without Malcolm', I would have understood perfectly."
"Me and Malcolm ... we always collaborated and bounced off each other," Angus said.
"You get an idea like Highway to Hell, and it would spring in you in the car.
"And you get into the rehearsal room, and I would pick up the guitar, and I would go, 'hey, Mal, I've got this'.
"Mal hops behind the drums, and you know, I'd start playing. I'd go, 'right, I've got the chord: Na Na Na'.
"And I'm going, 'Mal ... now?' And he's going 'now?' And I'm going, 'Yeah', and then boom, pah, boom - he did the beat.
" couldn't think of another chord and just thought, 'Oh well, hit E ...' Bam!
"And then, you know, just strum the first thing you hit for a chorus idea, and I went, 'Well, that's basically the idea'.
"And Mal said, 'have you got a title for it?' And I said, 'not yet ... I'll think on it'.
"Went to the toilet, had a cigarette, came out and said, 'I've got it! Highway to Hell'."
Malcolm Young said 'keep making music'
In the end it was Malcolm who decided for them.
"He said 'keep making music'," Johnson said.
"Without any of that sympathy stuff, you know, there was none of that stuff around.
"And he just said to keep doing it. That's the way he talked - [he was a] straight shooter.
"So we did, and it was great."
It is not the first time AC/DC has had to fill a massive hole.
It might have ended for the band in 1980, with the death of singer Bon Scott.
Angus and Malcolm decided to keep going.
They hired Johnson, a little known singer from the north-east of England, and went on to become one of the most successful acts in history.
"I didn't feel like I was an outsider just jumping in," Johnson said of starting with the band.
And he says Malcolm's replacement, Stevie Young, feels the same now.
"What will happen to him is the same thing that happened to me," Johnson said.
"The band just made us feel welcome straight away.
"You know, as soon as he got to the studio we just put our arms around him and said 'how are ya', kiddo?', and he was off."
Stevie Young is not entirely unfamiliar with the band – he is Angus and Malcolm's nephew.
He has played with the band before, on a tour in 1988, while Malcolm took time off to deal with his alcohol addiction.
Now, he is back - presumably for good.
"[Angus] said, you know, 'Stevie is there, I think we can do it' and I think we did," Johnson said.
"Yep, you've got to give it a shot. Your best shot," Angus agreed.
