Called in as a replacement FO for an STC's line check with another STC in the 3rd crew seat to being told that
Captain ZZ "it's Captain XX line check and not yours, we won't be asking you any questions"
Then getting questioned by Captain XX to which the reply was
"the answer is 'B' or 'C' if B is wrong "
After 4 or 5 questions and getting same answer, he gave up and didn't ask anymore after that.
Capt ZZ in back was quietly laughing