Jason Cosway is a character portrayed by Brennan Elliott in the ABC crime series Castle. Jason was the husband of Ashley Cosway, and the "dance partner" of Eric Marx in a murderous pas de deux, as he eliminated Marx's father-in-law Frank Anderson, while Marx killed his wife, Ashley. Each man made sure they had an airtight alibi for the time of the murder in their family, and only some typically Castle lateral thinking, and Eric Marx's relative gullibility, landed them both in jail.