After a trial has been decided, A says of Z, who was a witness in it—"I do not believe what Z asserted…

Exception 5 to section 356 of the BNS protects a good faith opinion on a witness's character only so far as it appears from his conduct in the case. Illustration (b) to Exception 5 says the remark based on A's own knowledge of Z is outside the exception, so the answer is "No, because A's opinion of Z's character is not founded on Z's conduct as a witness". By contrast, illustration (a) shows that calling Z's evidence so contradictory that he must be stupid or dishonest is protected. Good faith alone is not enough, and the exception does cover witnesses.