I'm looking for a specific fic my friend described to me where Dean works for the FBI and Cas is an ex-KGB agent who can no longer speak and talks to Dean via computer and sign language and acts as Dean's tech support on cases. There was one scene where Cas tries to tell Dean via earpeice about his theories on Snape in Harry Potter and the Half-Blood Prince. Dean and Cas were living together but it wasn't really an established relationship until later on in the story.
Edit: Found! Link is in comments. Thank you so much.