Just reading this piece, he talks about writing pages and pages about his character's bio, the dedication definitely shows in the movie:

Martinez: Did you create a character that we don't see in the film? A bio, so to speak. Grant: Yes. The older I’ve gotten – the more acting I've done – it’s almost obsessive. So, yes, there’s hundreds of pages of biography of this guy.

Martinez: Wait, wait. Hundreds of pages that you wrote? Grant: Yes. Yes. But while I’m writing them, I’m also connecting with the director, the writers saying, “What do you think of this? What do you think of that?” But a lot of it’s just me. And I prefer to keep it secret as well.

Martinez: Why do you do that? Grant: There’s two reasons. One, I have this belief that this intense marinade in the character and background somehow makes the character richer on camera. And the other is that I’m so nervous about acting, especially as a new film looms towards me on the calendar that just doing four or five hours a day, every day, for weeks and months, calms me down.

Martinez: So Mr. Reed is charmingly creepy. Grant: Yeah.