"I remember, I said something to her at one point about that, and she was like, 'oh,' like she didn't even know I was in a Marvel movie," Stan said with a laugh during a Q&A panel at Wizard World St. Louis earlier this month.
Stan went on to praise Robbie, who plays the Joker's crazed lover Harley Quinn in Warner Bros.' DC Extended Universe.
"She was definitely my favorite part in Suicide Squad," Stan said. "I thought she was great. Such a great character, too. DC has great characters."
Robbie received a Best Actress Academy Award nomination for her portrayal of disgraced ice skater Tonya Harding in I, Tonya. Stan plays Harding's ex-husband, Jeff Gillooly.
Stan admitted during the panel he would "love" to one day adopt the mantle of Captain America, a role currently filled by Chris Evans.
Stan said he doesn't know when that day would be, but it would be "a very different" Captain America.
"You wouldn't be able to have the same Captain America as you have right now because he's a different guy," Stan said of his Bucky Barnes, a World War II soldier and Captain America's best friend.
Bucky was brainwashed and converted into a ruthless assassin by terrorist organization Hydra, and was entered into stasis in Wakanda following the events of Captain America: Civil War.
"There's this issue where we've got to get him to be trustworthy enough for them to give him that responsibility, you know, to fill those shoes, and those are hard shoes to fill," Stan said.
"So I think it's possible, I really do, but it just has to make sense and we might need a little more time."
The former Winter Soldier will be reawakened sometime during the events of Avengers: Infinity War, where Bucky can be seen standing shoulder-to-shoulder with Captain America and Black Panther (Chadwick Boseman) in the blockbuster's trailers.
A new 30-second TV spot released during last Sunday's Super Bowl 52 offered fans a fresh look at the revived Bucky and his updated prosthetic arm.
Avengers: Infinity War opens May 4.