Production on the movie had already finished when Spacey was faced with several accusations of sexual misconduct, effectively ending his career.
He was swiftly replaced by Christopher Plummer, who reshot all of the scenes at very short notice.
All The Money In The World has already been released in the US, just weeks after the reshoots, and is out in the UK this weekend.
Scott told BBC Radio 4: “Clearly to do that was very uncomfortable.
“I was completely taken by surprise, but honestly, within an hour, my commercial head starts to click in and I realised I had to do something about it immediately.
“Christopher Plummer had always been there, almost alongside Kevin from the start as the possibility of Getty, so I went straight to Christopher.”
Scott was refreshingly honest when asked whether it was a commercial or moral decision to axe Spacey.
“That’s tricky, I think it was a little bit of both. But moral is a word we could talk about for the next two hours, okay?” he admitted.
“So I’d like to say, initially it was a commercial decision, which would be a little hand-in-hand with a moral decision.
“It would not have gone out if I hadn’t done it. It’s that simple.”
All The Money In The World is out today.