Think of 'impassive' as having control over facial reactions in emotional situations. Even when criticized, her face remained impassive.
Pro Tip 2/3
Not Cold-hearted
'Impassive' doesn't mean a person doesn't feel emotions, just that they don't show them. Despite his impassive exterior, he was deeply affected by the story.
Pro Tip 3/3
Professional Settings
Being impassive is often seen as professional, avoiding bias in serious situations. The judge listened to both sides with an impassive demeanor.