Just a suggestion....

Nathan Fillion

James REad

Every once in a while, the writers on Castle will tease viewers with the topic of Castle's unnamed/unknown father. The show's now in its fifth season and I think it's time they address it. (I mean, they ran that over-the-top conspiracy subplot involving Kate's mother through four seasons but it looks like it's finally been put to bed.) The actor I nominate to play Castle's dad:  James Read.  Doesn't he look like Nathan Fillion? They even have the same smirk/smile.