"Preacher" was a fantastic graphic novel by the equally fantastic Mr Garth Ennis about a preacher who had been "gifted" the power of the voice of God; that is to say, when he used the voice on someone, they obeyed his commands.  He was also, with or without the voice, something of a s**tkicker  when the occasion called for it.  Following a crisis of faith, Jesse Custer  was traveling the length and breadth of America to find God and bring him to task for abandoning the human race. His traveling companions were his ex-girlfriend and an Irish vampire. 

The comic book (being an Ennis creation) is one of the most ferociously blasphemous, rude, brutal, filthy things that you could imagine; it is also a fabulous examination of faith and religion, of family, friendship, loyalty and love, and of the state of the human race.  IF it makes it to TV or film in as intact a form as possible (which is doubtful for the reasons listed above) then, yes, yes, oh dear God, yes, please give Fillion the role of Jesse. (For Tulip, his girlfriend, Katee Sackhoff; for Cassidy, David Tennant would be left-field but choice.)