The longtime friends first starred in 1997's My Best Friend's Wedding before sharing the screen a second time in August: Osage County, which came out in 2013.
ET's Keltie Knight caught up with the actors at the premiere of their third joint project, Amazon's upcoming series, Homecoming, which will be available for streaming on the platform Nov. 2.
"He’s my pal," Roberts, 50, gushed. "It’s been such a long time we’ve been together, this is our third thing together, and... it’s one of the reasons why being in the business so long is so fruitful, when you can return to relationships."
Mulroney, 54, wholeheartedly agreed, calling Roberts "exquisite."
"We’ve remained friends all these years and so, it’s not just working with one of Americas greatest actresses, most endearing beauties, it is that I’m working with a friend of mine and these are really long, really beautifully written directed scenes," he said of Homecoming.
This isn't the first time Mulroney sang Roberts' praises. Back in 2016, Mulroney told ET that he'd "walk a mile on broken glass for her."
"It doesn't always happen with every person that you love and work with -- you kind of go your separate ways, but we, against all odds, managed to maintain a great close friendship," he said at the time. "...She's just one of the most genuine friends I have. She's a really generous person -- great sense of humor and really caring. Great family, and she's been a good friend and role model for me."