It's impossible for us to imagine Castle without Nathan Fillion in the title role, and it turns out Fillion felt the same way after reading the Castle pilot script several years ago.
When asked about how he landed the role, Nathan told Back Stage, “I had one of those deals with ABC. They put a stack of stuff in front of you, and Castle just happened to be the last one I read. I had a meeting with the producers, and I literally told them, ‘Stop looking. I’m right here.’ I don’t typically do that.”
As for what drew him to the Castle script in the first place, Nathan revealed, “I think that I enjoy watching characters who are flawed. I used to want my characters to be perfect and really cool and likable, and I learned quickly that they’re not relatable and really impossible.”
And as they say, the rest is history!
Source: Back Stage