The most common and most likely answer to this crossword clue is the 5 plus 3 plus 5 letter word UNDER THE KNIFE
I've seen this crossword clue in The Guardian
U
N
D
E
R
T
H
E
K
N
I
F
E
Buddy explains!
The straight part
We know that "different role in theatre" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case different role in theatre = under the knife.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.