Using 'in public' can emphasize the contrast between someone's private and social behavior. He seems stern, but never cries in public.
Pro Tip 2/3
Social Norms
The phrase often implies behavior against what is socially acceptable or expected. Eating with your hands might be frowned upon when done in public.
Pro Tip 3/3
Privacy Context
'In public' can highlight personal matters not usually shared with strangers or acquaintances. She was reluctant to discuss her health issues in public.