The "Black Panther" star was secretly fighting colon cancer at the time of the movie's shoot in 2018, but Miller insists he never let on he was ill even though he was so thin and tired in between takes on set.
In an interview with The Sunday Times Culture magazine, she said, "I would never have known he was sick to the degree he was. But I could tell he was really tired in a way that now makes sense."
"It never affected the work. But he was very thin. Thinner when we went back to reshoot. It is astounding he was at that stage of a battle and we made that film."
Miller also recalls how emotional Boseman was when they wrapped filming and were saying their farewells.
"I remember when I said goodbye, the last time I saw him," she shared. "In reflection, which is often the case, it felt like a real goodbye. And I remember being really moved, but also really confused."
"He had tears in his eyes and it was a hug and I felt it was the sweetest thing ever, and never saw him again. But it makes sense of course. You put the pieces together in the aftermath."
Miller had previously heaped praise on Boseman, who died last summer (20), for giving up part of his own salary for "21 Bridges" to ensure she was better paid and it's inspired her to continue fighting for what she's worth.
She said, "For many years I underestimated myself to such a degree I would've accepted anything."
"But Chadwick's gesture and the way women in my industry are talking about (pay equality) is empowering. I'd feel embarrassed now to not fight for myself. But it can take me being put into that corner to do something."