Jonathan LaPaglia is an Australian actor who plays John Knox on Castle. LaPaglia is best known for his roles on New York Undercover as Detective Tommy McNamara, on the TV series Seven Days as Frank B. Parker and on The District as Detective Kevin Debreno. LaPaglia is also the younger brother of actor Anthony LaPaglia and he has been married to actress Ursula Brooks since 1998. He is currently (2014) to be seen in Love Child.