ET spoke with the 48-year-old actress on Sunday while she was promoting her new film, The Glass Castle, where she opened up about their decades-long friendship. On Saturday, Watts Instagrammed a sweet picture of the two hugging, writing, "This woman is everything to me."
"Well, it was sort of a belated birthday celebration for her because we weren't together at the time, and I was in Europe and couldn't get to see her, so I'd been saying I wanted to do something for her, or with her," Watts explained of their get-together, which also included producer and fellow Aussie Bruna Papandrea.
Aside from celebrating Kidman turning 50 on June 20, the accomplished trio also toasted Kidman's recent Emmy nomination for Best Actress in a Limited Series or TV Movie for her role in Big Little Lies.
"It was a celebration of birthdays and Emmys and and all things, and just, like, great, great friends who have great powerful effects and love -- effects on each other and love for one another -- and we've been through stuff together, and it was time to just celebrate," Watts explained.
Watts also talked about both women finding great projects on television -- Kidman with HBO's Big Little Lies, and Watts with Netflix's Gypsy, in which she plays a New York therapist.
"Yeah, it's a great thing that's going on there with TV -- fantastic female-driven stories that are being made," she acknowledged. "Obviously, as actors we're really happy and, you know, it doesn't really matter the format, it's just that you get to play these characters with love, and hope that the stories connect with people."
As for her latest film, The Glass Castle -- in which she plays the mother in a dysfunctional family of nonconformist nomads -- Watts shared she was deeply affected by the final product.
"I was wrecked," she said about her emotional reaction to the film, out Aug. 11. "I had to run out to the bathroom and get myself together before anyone saw me or I could talk."
Watts is of course a mother to her own kids with ex-partner Liev Schreiber -- Samuel, 8, and Alexander, who turned 10 earlier this month.
"Yeah, I can't believe it," Watts said about her eldest son hitting double digits. "I mean, it's such a cliche to say, but it's come around so fast. I can't believe that 10 years ago I was, you know, carrying that big ol' baby, and here he is as a little man in the making."