Asked at a recent New York press conference what it was like to work together again, Roberts told United Press International: "Dermot and I have been friends since 'My Best Friend's Wedding.'
"We became great pals then and when he called me and said he got a part in this, we squealed like little girls -- both of us. We were so excited for each other and to be back together," Roberts recalled. "Even when he had a day off, he would come run lines with me before I went to work at 6 o'clock in the morning. He'd trade [the service] for a good cup of coffee. It was just the whole time feeling so supported by my old friend. He would come to work and watch scenes that I was in and support me in that way. It was beautiful to have each other in that way. Especially when we are all away from our families and forging new relationships, it was nice to have my rock steady there. It was great."
"I'd start my day with coffee with Julia and because I know her and love her, we would whisper in each other's ears on the set and then I'd kiss her good night," added Mulroney, who was sitting alongside Roberts at the press conference.
"That's not going to go over well in print," Roberts joked. "Let's get that straight, people!"
"On the cheek! On the cheek!" Mulroney clarified.
Based on Tracy Letts' play and directed by John Wells, "August: Osage County" is about what happens when three estranged sisters -- played by Roberts, Juliette Lewis and Julianne Nicholson -- return to their childhood home in Oklahoma with their respective husbands and boyfriends -- played by Ewan McGregor, Mulroney and Benedict Cumberbatch -- after their book-loving father disappears and their overbearing mother is diagnosed with mouth cancer. Meryl Streep and Sam Shepherd play the parents, while Margo Martindale and Chris Cooper play the sisters' boisterous aunt and kindhearted uncle.
The movie marks the second time Streep and Cooper have shared the screen. They previously co-starred in 2002's "Adaptation." Streep and Martindale also appeared together in the films "The Hours" and "Marvin's Room."
"I've worked with Margo before, but never in such a substantial way. The thing about this piece is -- and I wish Ewan and Benedict were here -- because we were all absolutely integral to this thing working or not," Streep told UPI when asked about reuniting with past co-stars for "August."
"Chris' character, I felt, was someone that he would imbue -- and he did -- with his enormous humanity and compassion and I knew the audience would love him and I knew they would hate me in equal measure and that is the story. It's a balance of all these characters. ... It only works if we are together and we were so together.
"This person," Streep said, gesturing to Martindale, who was sitting near her at the press conference. "When I have a speech [in the film] about her having my back, I really always, always, always felt that because she made me feel that way. I felt like we were very lucky to have each other in making this thing. That does have to do with the way John set it up. Because how do you put a family together? You don't get a vote about who is in your family, but John was like God and he put this group of people together and thought, 'This will get messy.' It was masterful."
"August: Osage County" is scheduled to open in theaters Dec. 25.
Swim Week Miami Beach 2014 [PHOTOS]