uncastable

adjective

a) That cannot be cast (filled with, or allocated, a theatrical role).

Colleen Dewhurst, though in many ways wrong for the virtually uncastable Josie, is as lucid and luminous...

b) That cannot be cast (converted to other data types).

...that he would, like many actors, become so stereotyped in the public mind as to be uncastable in other roles...