There are many times. Death is one of those times as is when someone is cornered by the truth. Others may be on their best behavior until they feel no one is paying attention or that they will not be caught saying or doing something negative. Abusers will often act one way to gradually isolate their victim and once isolated that is when they show their true colors.
But in all honesty, even when things are perfect or simply good, we can get a glimpse into another person's true colors if we are willing to be fully open to it. It's not uncommon for people to not even allow anyone to speak a negative truth about someone else, they are not open. However, they may be among the many people who say in hindsight that there were clues, glaring signs that even others saw, etc. You say it best when you say "pay attention."