In my opinion, people watching is an essential skill for any and every writer to have. By taking some time to watch how other people behave, you gain valuable insight into other people. How they act, their habits, quirks, and a lot of other things.
Try to watch specific things. When you sit at a restaurant and look around, watch expressions. When you're out at the park, watch body language. If you mix them together in long stretches, you end up forgetting a lot of the details that you want to look for.
The distance between two people can say a lot about how they feel. If two people are standing within a foot of eachother, they are very familiar with each other. Perhaps lifelong friends or romantic partners. A couple feet usually indicates that the people involved know each other, but won't feel comfortable being too close. When that personal bubble is crossed by one party, you can usually see the discomfort on the other's face.
And distance futher than a couple feet is reserved for aquaintences.
The direction people face when talking to each other also gives indication of their feelings, though slightly more subtle than the space between them. Side by side and face to face mean completely different things. When one person is leaning outward from the other, it usually means they aren't interested in the conversation.
Expressions can give you a lot of insight as to how a person is feeling. How does a person's eyes, mouth, ears, cheeks move when they are feeling certain emotions. Eyebrows usually come together when they don't understand something. They move down in the middle when someone is mad and down at the outsides when someone is sad. When a person is happy (really happy) they get creases at the edges of their eyes, their eyebrows raise slightly, their ears move back, and their cheeks lift. Paying attention to these details and using them in your writing allows you to convey emotions without having to tell the reader what the character is feeling.
When you watch people and you see how they are feeling and acting, try to come up with little stories about why they are acting the way they are. Just try not to be too obvious. It makes things more interesting and it can give you ideas when you are writing.