Theoretically opera prepared lead characters to naturalistically perform parts (2,5)
I believe the answer is:
on paper
'theoretically' is the definition.
(synonyms)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for on paper that I've seen before include "In writing" , "Where one might write" , "As written - in theory" , "Theoretically" , "Hypothetically" .)