It's easy to put on a smile and go about your day, some people are able to mask their mental illness easily. I know people who have put on a good face, but they are secretly struggling with their mental health. Oftentimes they feel ashamed to ask for help, but no one should feel ashamed to ask for help, especially if you need it.
Depression works on all of us differently. Some of us are more open while some of us bottle it in. Sometimes you can't tell, which is why you need to talk with your friends, family, keep them in your graces. Sometimes just being there helps.