Correct me if I'm wrong, but I heard that one reason why certain directors can only use certain characters (i.e. those they created) outside of the "universal" cast (Bugs, Daffy, Elmer , Porky, Sylvester) could be because they were restricted from using other characters. I'm not sure if it's true, especially since McKimson stated in a 1971 interview with Michael Barrier that no one could stop them from using different characters, using Goldimouse and the Three Cats (Sylvester Jr.), Dog Pounded (Pepe le Pew), the shorts starring Sylvester and Porky and the three Chuck Jones shorts featuring henry Hawk as examples.