Greta Gerwig starred alongside the pregnant Portman in the romantic comedy earlier this year, and the two have since become good friends.
"I think if everything else she does in her life is an indication of what a great mum she'll make, she's going to be the best mum in the world," Gerwig told People at last night's Independent Spirit Awards.
"She's a smart lady," Gerwig added. "She's a lovely girl to pal around with. It's been really great to know her."
Gerwig lost out to Portman at the Independent Spirit Awards in the 'Best Actress' category.