It was recently revealed by director James Wan that Randall Park will be a part of Aquaman, and it sounded like he was brought in specifically for the reshoots. As it turns out, that wasn't the case, ad Wan clarifies that Park was always meant to be in the film.
Wan replied to a fan that was curious about Park's casting:
Hiya Grace! Love your stuff, so I'll speak up. I didn't add a new character now, Dr Shin was in the script since day one. I had cast Randall a year ago in Aust (I'm a big fan of his/love FOTB), but I didn't shoot his stuff till now. Hence, it's literally "pickups" w him. Cheers.
Park is said to be playing a Dr. Shin, a character in the comics who had helped mentor Aquaman to fully understand his powers. We don't know how big of a role Park will play in the film, but he's a face that's always welcome on every screen.
It's worth noting that Park has also been cast in rival franchise Marvel's Ant-Man and the Wasp. He'll be playing a character named Jimmy Woo who is an FBI Agent in the comics. We don't have a look at him in that film yet, though.
Catch Randall Park in 2018's only DCEU movie, Aquaman, which comes out Dec. 21.