Renée Zellweger and Bradley Cooper
Jackson Lee/Splash News Online
"They get along really well and just kinda click," a source tells PEOPLE in the upcoming issue. "Bradley likes the speed of everything. He likes that Reneé is low-key."
The pair, who costar in the upcoming thriller Case 39, recently spent time in Spain together, then returned to the States, where they each went their separate ways – briefly.
On Saturday, Zellweger attended the premiere of her new movie My One and Only in Wainscott, N.Y., while Cooper was a guest at Alyssa Milano's wedding in New Jersey that same evening.
But the cozy costars soon met up again. Sunday night, they attended a private party together in New York City, then were spotted at a Starbucks in Philadelphia on Monday.
As for why Cooper didn't come to Zellweger's screening and she didn't accompany him to the wedding, the source tells PEOPLE: "They're not ready for red carpets. They don't want to be public just yet."
• With reporting by JENNIFER GARCIA
