Boseman died in August aged 43 after having colon cancer for four years.
Miller, 39, said the actor had topped up her wages to what she had originally asked to be paid for the crime thriller using his own salary.
“We became close friends and I didn’t really want to bring it up, so I wrote him a letter to thank him.
“He said, ‘You got paid what you deserve and what you are worth’.
“That was so validating from a man I respected so much. It’s that kind of behaviour that moves the needle – there is a lot of talk but not much action.
“He had grace and dignity – it wasn’t flashy, he did it because it was fair.”
21 Bridges saw Boseman play a New York detective involved in a manhunt for a pair of murderers, while Miller played a police officer who worked alongside him.
The actress has also starred in Wander Darkly, which she described as “a very real surreal love story”.
She added: “It was complicated to make, and my character has no idea where she is or what has happened to the world she is living in.
“So, with it coming out now, I think people will get it. It is remarkable.”
BBC One’s The Graham Norton Show airs on Friday at 10.45pm.
— to www.standard.co.uk