The actor, 43, stars in the series as Marc Spector, a man who harnesses the power of the ancient Egyptian gods to fight crime.
Marc has dissociative identity disorder, which manifests in four different personas, one of which is a British man named Grant.
“And so I had a meeting with Kevin, I said, ‘This is how I’d want to do it.’ And he said, ‘Yeah, OK, go for it.’”
He added: “It’s funny, because afterwards he told me they didn’t know what the hell I was doing. And they weren’t sure it was going to work at all.
“But you know, in the end I’m glad we did that, because everyone says it kind of makes the show.”
Isaac also admitted that he had “reservations” about taking the lead role in Moon Knight after finishing work in Disney’s Star Wars sequel trilogy, in which he played pilot Poe Dameron.
Moon Knight begins on Disney Plus on Wednesday 30 March.