The "Sin City" star hung out with the former Batman actor on the set of the drug drama series, even though she doesn't have any scenes with him, and admits she was impressed with his efficient way of working.
"He blocks shoots," she tells "Good Morning America". "He came in and did his entire performance in five weeks."
And Rosario is convinced Keaton will pick up an Emmy for his performance on the show - just like he is.
"(After) episode four, he's like, 'Gimme my Emmy! Give it to me!' " she recalls. "He's so good. What a G - original gangster."
Rosario also reveals she came up with a nickname for her co-star, using some of his most famous roles: "(I called him) Michael My Forever Batman Beetlejuice Beetlejuice Beetlejuice Keaton... It rolls off the tongue! I'm obsessed. I think he's such a brilliant actor."
Based on Beth Macy's bestselling book, "Dopesick" is an ambitious, harrowing, and compelling look into the epicenter of America's struggle with Opioid addiction.
It takes viewers from a distressed Virginia mining community, to the hallways of the DEA, and to the opulence of "one percenter" Big Pharma Manhattan. The unsparing yet deeply human portraits of the various affected families and their intersecting stories hold up a mirror to where America is right now, while shining a hopeful light on the heroes battling the worst drug epidemic in American history.