So close, for example, that one of the first things Downey, 51, said to Evans, 34, at the start of their photo shoot for PEOPLE was "You look straight ahead – I'm going to give you a smooch."
But that was far from the first time he's shown affection for his Captain America: Civil War costar.
Downey reveals that he ably came to Evans' aid during filming when his fellow superhero went a little too far with a particularly challenging stunt.
"I came to set to start shooting and they show me a couple minutes [of footage]. Like, 'Here's what we've been doing.' This is a shot where Cap is holding this big [helicopter] and I say, 'You know what? All the things he's done, that's the one that was going to injure him.' I knew it," Downey says. "Because they're saying, 'Flex and hold it!' It wasn't the other 3,000 stunts, it was that."
For more from Robert Downey Jr. and Chris Evans, pick up the latest issue of PEOPLE, on newsstands Friday
"I did, my arm did get all messed up," admits Evans about shooting the scene, which is a memorable moment in the film's trailer and in a teaser clip.
Fortunately, it was Nurse Robert to the rescue.
"I went to [Downeys] trailer and he put these things all over my arm. I did actually mess my arm up doing that helicopter thing," Evan says.
Downey describes the "things" as electric guns, which we're assuming are the key components to some sort of high-tech pulsating muscle massager.
"[The arm] is still a mess, actually," says Evans. "I need your thing more."
"I've got it ready for you," replies Downey.
Captain America: Civil War flies into theaters Friday.