Sam Heughan and Caitríona Balfe don’t remember life without each other after more than a decade on Outlander.

Before the series finale airs on Friday, May 15, the actors tell PEOPLE that their shared sense of “humor” is among the things they’ll “miss the most” about working together.

Heughan, 46, says even in “the darkest of times,” Balfe’s humor never waned. “We had a lot of fun, and there were some tough days and hard subject matter, but there was always a good sense of humor,” he says.

“I think we’re very different people, but we have a very similar funny bone,” Balfe, 46, says. “The same things crack us up.”

She praises Heughan as “one of the most easygoing, generous, just lovely people to be around.” Plus, “There’s no ego with him on set.”

“What I miss is, in the morning, in the trailer, you rock up, and you’re tired, and we just sit there, and we have a coffee and talk s—,” Heughan admits. “And then run some lines and then probably [talk] more s—.”

“It was lovely, it was a nice way to start your day,” he says of his and Balfe’s morning ritual.

They’re not concerned about their friendship slipping now that they won’t be reuniting annually on set. In a “weird” way, Balfe says, “We’ve actually found a way of seeing more of each other.”

“I think our friendship is now… I don’t know, maybe you take it for granted, because you are seeing each other all the time, so then you don’t make the effort to see each other outside of work as much,” she says. “But now we have to make the effort to see each other, and we have, and we do.”

The cast as a whole has remained tight. “For all the cast as well, it’s been so nice seeing them more occasionally, meeting up in groups of us — there’s quite a few of us — and it’s so nice,” Heughan says. “There’s a great rapport and a great camaraderie there.”

“We’re all still really close,” Balfe agrees.

The Irish actress recalls one of her earliest memories with Heughan in 2013, after they were cast in the lead roles but before they started filming the series.

“We had this walk before we started filming in Hyde Park, and we said at that time, ‘Look, we don’t know what this is going to be, but no one else is going to have the same experience as you and I are going to have,’ ” Balfe says. “I think that that bond will sustain us for the rest of our lives, because no one else will have had this experience in the same way that we’ve had.”

Heughan adds: “It’s changed our lives completely.”

The series finale of Outlander premieres May 15 on Starz.