In fact, Ethan admits he was incredibly irritated by the late actor while they were filming the 1989 flick.
During a Q&A at the 55th Karlovy Vary International Film Festival, Ethan opened up about his relationship with Robin.
"I thought Robin hated me. He had a habit of making a ton of jokes on set. At 18, I found that incredibly irritating. He wouldn’t stop and I wouldn’t laugh at anything he did," Ethan explained.
He detailed one joke in particular, where Robin called Ethan "intimidating" after reciting a poem while filming a scene.
Ethan says that as he's continued on in his career, he's realized Robin's comments weren't a joke at all, and instead, something he relates to now.
"As I get older, I realize there is something intimidating about young people’s earnestness, their intensity. It is intimidating — to be the person they think you are. Robin was that for me," Ethan continued.
And while Ethan thought that Robin disliked him at the time, he soon discovered that wasn't the case at all when he got a call from the first agent who wanted to sign him.
"He called, saying, 'Robin Williams says you are going to do really well,'" Ethan revealed.
Unfortunately, Robin passed away in 2014, but Ethan is forever grateful for Robin's support in his career.