Ifans reveals he was an impressionable teenager when he first learned about fellow Welshman Marks - and became an instant fan and pen pal.
The Notting Hill star recalls, "He was a Welsh speaker and I saw him on the news and he was supplying the world with pot, which, at the time, I guess, was appealing to me. I would write, 'Hello, I'm in school. I'm bored. This is more boring than prison. I know how you feel...' - that kind of teenage stuff.
"When he came out of prison and wrote Mr. Nice we had a conversation when I was a teenager and I said, 'One day when they make a film out of this, can I play you?,' and lo and behold it happened last year.
"It's come full circle completely, so it was really an amazing experience, playing a friend and getting to discover stuff. He had a great life but a tough one as well. Of course now I've seen - and he's seen - the folly of his ways."