No doubles for Streep, Roberts
The pair, who play mother and daughter Violet and Barbara, threw themselves into the stunt, which sees Julia's character attacking her mother in a fit of anger.
"We had stunt co-ordinators and stunt performers to fill in for them, but neither Julia or Meryl wanted to use them," he said.
"I was a little nervous because Julia attacked Meryl, and they went slamming down to the floor. It was more physical than I thought it was going to be. They were both very committed to attacking each other. They really did it so it looks good on screen," he added.
Julia had her doubts about the scene prior to filming.
"Julia was very worried - she kept saying, 'Am I really going to attack Meryl Streep?'," he said. "Leading up to it, she was very nervous about it, and then the day we get to the set, she said, 'I get to strangle Meryl today'. She was very excited."
The filmmaker also revealed that Meryl had her concerns about the role, which she initially turned down.
"She had reasonable doubts about the difficulty of playing the role. The role is, on a day to day basis, a difficult place to be," he explained.
"She's bitter and sarcastic, a holy terror to everyone who comes into her path. Meryl is actually a personable, kind, gentle person and so day after day, she was attacking all these people that she likes."
He added: "And she had to smoke - she's a chain smoker in the piece - and she doesn't smoke. Physically, that was difficult. A lot of days, we would finish and she would look at me and say, 'This is really hard'."
:: August: Osage County is in cinemas now.