Used during events where polite behavior is expected, like weddings or formal dinners. She knew to be on her best behaviour during the awards ceremony.
Pro Tip 2/3
Expectation
Suggests that there's an expectation to act better than usual. The children were reminded to be on their best behaviour when the guests arrived.
Pro Tip 3/3
Impression Matters
Important when first impressions count, like meeting someone's family. He was on his best behaviour when he met her parents for the first time.