"He has an incredible skill of the best -- this is going to sound crazy -- baths possible," Susan told ET on the blue and red carpet. "He and our [4-year-old son Exton] take the most adventuresome, weirdo baths."
"Yeah, fun baths," Downey agreed.
While the action star gets into adventures with his son at home, his film alter-ego gets into an epic battle with his former ally, Captain America, in Marvel's latest blockbuster.
In the movie, a rift forms between Iron Man and Captain America (Chris Evans) due to political pressure to install a system of accountability for the Avengers.
Evans puts on a brave face in the film as his character goes toe to toe with his friend-turned-foe, but last night he found his knees knocking at the gravity of how much the film matters to moviegoers.
"When you see fans come out and when you see how enthusiastic people are, you're kind of shaking in your boots about it," Evans told ET. "It's nerve-wracking, but it's exciting."
Captain America: Civil War opens May 6. Check out the trailer below!