The 58-year-old actor – who previously battled addiction – was fired from the show in 2011 after publicly lashing out at showrunner Chuck Lorre and Charlie's was having difficulties for some time, according to director Jamie Widdoes.

Speaking on Vice TV's 'The Dark Side of the 2000s', he said: "We all just would soldier on, and we would try and separate whatever was going on with Charlie outside with our job, which is to make a television show.

"We were all genuinely worried about him. And Charlie [would just say], 'Don't worry I'm fine.'

"Whatever he was battling outside was starting to not just leak in, it was starting to flood in."

Despite Charlie's assurances, he soon stopped showing up for work.

Widdoes added: "The word came down that Charlie's stand-in's going to do the Monday table read.

"And what happened, as we got more and more into season eight was that the Monday table read started to become the Tuesday rehearsal as well."

Meanwhile, Gabriel Solana, Sheen's personal make-up artist, claimed the actor struggled with the monotony of the show.

Gabriel said: "I knew that something in Charlie wasn't already too comfortable in the show because he'll come to me and say, 'I don't know man, I'm not feeling this. This is a little bit monotonous for me already. What's next for me?'"

However, Charlie has managed to turn things around and is now sober.

Gabriel said: "He's completely clean today. He doesn't even smoke cigarettes… I never thought he was going to do that."