The actress told Marie Claire that Streep was "even cooler" than she thought she would be.
> Julia Roberts: 'I've decided not to Google myself'
She explained: "I had many imaginary conversations with Meryl about working together… When a person goes from being on a pedestal in your mind, to even cooler, better and more fabulous up close, you get to understand them a little more intimately."
The actresses star alongside Ewan McGregor, Benedict Cumberbatch and Chris Cooper on the project, which has been adapted by John Wells.
August: Osage County - originally a Tracy Letts play - centres around the reunion of a dysfunctional family who return to Oklahoma for a funeral.
The 46-year-old also revealed that she would feel different to the last time if she won a second Academy Award.
"Listen, every actor wants to win an Oscar, full stop. It doesn't mean you try to design things around how to accomplish that because that's impossible.
"Now that I'm a grown-up person and have things of such greater value to pin my idea of joy and fullness on, if something like that were to happen again, my feeling of thrill would be defined in a completely different way," she added.
Roberts won the Oscar for 'Best Actress in a Leading Role' for her performance in Erin Brockovich in 2001.
August: Osage County is out now in the US and will receive a limited UK release on December 26.
The full interview with Roberts appears in Marie Claire's December issue, available from November 19.
Watch a trailer for August: Osage County below: