John Slattery Praises 'Great Guy' Jon Hamm and Fiancée Anna Osceola: 'They're Lovely' (Exclusive)

John Slattery is raving about his pal Jon Hamm and his new fiancée Anna Osceola!

Speaking to PEOPLE on Monday at the world premiere of Maggie Moore(s) starring Hamm, the director says his former Mad Men costar is a "great guy" while expressing how happy he is for the actor's new chapter.

"I mean, they're lovely. Together, they're great people. I'm extremely happy for them," Slattery, 60, tells PEOPLE, who goes on to express how lucky he is in his nearly 30-year marriage to Talia Balsam. "So I couldn't be luckier and I hope he has the same luck as I did."

Balsam, 64, also starred in Mad Men as Slattery's on-screen wife in AMC's award-winning drama series, which aired from 2007 to 2015.

Hamm, 52, and Osceola, 35, who were in attendance together on Monday at New York City's Tribeca Film Festival, reportedly first met on the Mad Men set in 2015. They were spotted together on several occasions and were romantically linked in 2020. PEOPLE confirmed their engagement news in February 2023.

Hamm discussed his relationship with Osceola in a September 2022 interview with Howard Stern on his SiriusXM show where he revealed that he could see himself being married someday.

"This is all part of what I'm saying. This is another place in my life that I feel very settled and comfortable. I'm in a relationship right now and it's comfortable," he said, later adding that he's been able to sit down to start "really thinking about all that stuff that's made the relationship that I'm in now even more meaningful and opened up the possibility of things like being married, having kids, defining a new version of happiness, life, wellness."

"It sounds hokey and whatever, but it's real and for lack of a better word, it's what I'm working for. What else is there other than that?" he added before confirming that he is "very much so" in love.

In Maggie Moore(s), Hamm plays opposite Tina Fey in the small-town murder mystery.

"He's a great comedian," Slattery added of Hamm. "Obviously, we've watched him in Mad Men. We all watched him fully inhabit this incredibly complicated character, extremely smart. He's charming and funny."

The good friends also worked together in 2022's Confess, Fletch — also with Osceola — where Hamm notably gave back 60 percent of his salary so that the film could be completed after the team majorly exceeded budget.

Discussing Hamm's swagger in a 2022 interview with The Independent, Slattery shared Hamm's effect on people on set when dressed in his Don Draper suit attire.

"When Hamm walked into a room in that get-up, people would just go catatonic," the 60-year-old actor, who played Roger Sterling, told the online newspaper. "Guest stars would sometimes walk up to him and their lines would go right out of their heads."

Slattery added, "They just wouldn't know what to do. It happened on more than one occasion."

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Back on Monday's carpet at the Tribeca Film Festival, Slattery tells PEOPLE he "hit it off early" with Hamm on "day one" of Mad Men.

"We worked well together and we became friends, and that friendship grew and now it's, what 15 years later or something, 10 years later — our partners are friends ... he knows my kids and we spend time together in various places."