Bullock, who is mum to adopted son Louis, and MCCarthy, who has two daughters with husband Ben Falcone, grew close shooting the movie in Boston, Massachusetts last summer (12), and the Oscar winner admits she was shocked to learn they had so much in common.
She tells Parade magazine, "Having kids connected us on a deeper level. And the things we're obsessed with outside of being a mum are the same, too. Construction and house renovation... The minute they'd say, 'And cut,' we'd bolt to our little chairs, flip open our iPads, and look at magazines.
"(We'd say), 'I'm tearing down a wall; do you think we should do the window here?' We're kindred spirits in that world."
The stars brought their children to the set everyday and the Bridesmaids funnywoman reveals the trio acted like siblings after spending so much time together: "Even though he's tiny, Louis really played big brother. He would put his arms around their shoulders or go in front of them and block them if he didn't like something. He was running security."
The actresses have remained close ever since, with both families enjoying a holiday trip to Disneyland together and Falcone took little Louis out trick-or-treating last Halloween (12), and Bullock admits it's nice to have found such a lovely couple to spend time with, adding, "When she and Ben invite you into their circle, you're like, 'This is going to be great!' To see a couple that has fun, that supports each other - that's what I love about them."