Summarize:

Everyone is unique, and different people will have different habits and quirks. Pay attention to how they typically behave. When someone reveals that they like or dislike a certain thing, make a mental note about it. That information could be useful later, to help you be a better friend to them. People are often pleased to discover that you remember things about them.   For example, if your friend gets distracted by a display at an art supply store when the two of you are at the mall, make note of what caught her interest. She may be delighted if you buy her something similar to that for her next birthday. If your boyfriend turned quiet and looked uneasy when your friend told him details about a medical procedure, then you'll know he probably wouldn't enjoy watching a medical drama. Everyone is sensitive to certain things, whether they'll admit it or not. It's part of being human. By noticing what upsets someone, you can help avoid and de-escalate high-stress situations.   What scares them? What puts them on edge? What is important to them? What do they want to protect? People's pasts can reveal their habits, and the important experiences that helped shape them. However much you know about their past, keep it in mind, because it can help you understand what they do and how they feel today.   What are their favorite memories? Their favorite memories can reveal what they care about, and what they want out of life. What are the worst things that happened to the person? Whether the events were just upsetting or seriously traumatic, the person will probably try to avoid similar experiences in the future. Everyone is unique, and people with different backgrounds will react differently to situations. Especially if someone is very different from you, be careful about making assumptions. Here are a few of many factors that can influence a person:    Culture: Different cultures have different social norms. For example, eye contact is considered polite in most western cultures, but considered a sign of disrespect or aggression in some eastern cultures.  Gender: Men are often expected to be tough, and to avoid showing emotions besides anger. Women are often expected to be passive and easy to get along with. Watch how the person navigates gender expectations.  Disability: People with some disabilities (like autism and ADHD) have different body language, such as avoiding eye contact or fidgeting when listening. If you aren't sure what someone thinks or how they feel, it can be worth asking politely. They may be happy to clarify for you.   "I noticed you checking the time. Do you need to leave?" "I saw you wince when he raised his voice. Are loud voices something that bother you?" "I'm having a hard time reading you right now. Is something upsetting you, or am I misreading you?"
Notice patterns in how someone acts. Figure out what they like and dislike. Notice what the person is sensitive to. Keep their past in mind. Keep their differences in mind. Don't be afraid to ask questions.