Clooney and the 11-year-old actress play father and daughter in director Alexander Payne's family drama The Descendants.
Miller revealed to Parade that Clooney took on a parental role with the movie's young cast while filming the family drama.
"George made a great on-camera dad in the movie. On set, he is quite the prankster and made me feel very comfortable and relaxed since it was my first movie," she explained.
"He did not really give me any advice, but I learned from watching him. Just little things. I think the most important thing I saw was how he treated the people around him."
Miller went on to say that she was truly honoured to play Clooney's daughter.
"It was amazing. Nothing could ever beat it," the star commented. "George was such a cool person to work with. He really welcomed me. The first day on set, it seemed like I had known him all my life."
The Descendants is currently playing in limited release in US cinemas and opens in the UK on January 27.
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Watch a trailer for The Descendants below: