Green Mile actor Michael Clarke Duncan died of natural causes, a US coroner has confirmed.The 54-year-old died at Cedars-Sinai Medical Center in Los Angeles on Monday. He had not recovered from a heart attack he suffered in July.Meanwhile, tributes to the actor have continued to pour in.Bruce Willis, who starred with Duncan in Armageddon, called him "a great human being". He added: "I will miss Big Mike in a big way."Duncan had credited Willis for recommending him to The Green Mile director Frank Darabont.He was later nominated for an Oscar in 2000 for his role as a death row inmate in the film.Willis and Duncan went on to star in another three films together including The Whole Nine Yards and Sin City.Director Michael Bay, who gave Duncan his first major role in Armageddon, penned a lengthy tribute about the actor's time on set, which was posted on his official website."It was a great time I will always remember, how proud I was to watch him grow into an actor," he said."We found him in a gym. He cried at the first audition because he was so proud to audition for a Michael [Bay] and Jerry [Bruckheimer] movie, he just wanted to make his mom proud."We gave him the role in the room."But, Bay recalled, the star "sucked" on his first day in front of the cameras."I remember looking to Ben Affleck and thinking we might need to fire him."But I told him 'Mike, I hired you for you. I want the sweet, Mr Clarke Duncan I met in that room'. I said, 'the audience is going to fall in love with you'."He looked and smiled with deep voice and said 'OK.'"From then on out he became the most improved actor on the set. That was the award he got at the end of the film. Everyone loved him, his infectious spirit and great belly laugh."Actor Tom Hanks made similar comments about his former co-star. "He was the treasure we all discovered on the set of The Green Mile. He was magic. He was a big love of man and his passing leaves us stunned."Green Mile director Frank Darabont added: "Michael was the gentlest of souls - an exemplar of decency, integrity and kindness. The sadness I feel is inexpressible."Duncan is survived by his mother Jean, sister Judith, and fiancee Reverend Omarosa Manigault.The actress and reality TV star told Radar Online she was "devastated" by the actor's death."He was the love of my life," she added.Craig Harvey of the Los Angeles coroner's office said that Duncan's personal physician had signed the death certificate, and there was nothing to suggest officials need to review the case.