It was love at first sight for Kevin Costner.

Smitten right off the bat with his now 7-month-old Labrador puppy, Bobby, Costner introduced the adorable “newest addition to the family” on Instagram in February,

sharing: “I’m already in love with this special guy.”

The mutual affection — and shared personality traits — has only grown stronger since then.

“I’m like Bob,” Costner tells PEOPLE in his recent cover story. He explains that he and his canine buddy share the tendency to go from extreme “passivity” in their everyday lives to full-on locked-in mode, where they fire on all cylinders.

“Bob just gets in the car. He does not care where we go. Bob’s lazy; all he does is sleep. He just goes ‘woof’ at the door a few times. But if I put Bobby in the field, you see his IQ go up 30 points. He looks like an Adonis of dogs. He just knows exactly who he is,” Costner adds.

Costner recognizes he embodies that same quality. “I can be very passive in my life,” says the Oscar winner, whose epic Western Horizon: An American Saga hits theaters on June 28. “But once I commit to a movie, I’m relentless there. All they have to do is get me [to set], and then I know what I’m doing. I become like Bobby.”

“I’m not sure what’s going on, but you get me on a set, and my IQ goes up 30 points…. I know what I need to do. That’s the closest to watching an animal be exactly where they need to be and how they look in the wild.”

Bobby — who spent time on the set with Costner while his dog dad filmed the third of four planned chapters of the ambitious Horizon project — has one peculiarity the star does not share: a fondness for sleeping in the dishwasher.

“I have to push him out,” Costner says, noting Bobby has outgrown this favorite spot. “He’s too heavy now. He wants to get in there, and he’s not comfortable with me kicking him out.”

Noting that the quirk might have been adorable when he was a smaller pup, Costner says, “He is too big for it… He doesn’t look cute in it. He looks like a big towel in there now, a big white towel.”

For more from PEOPLE’s Kevin Costner cover story, please pick up this latest issue of PEOPLE on newsstands now.