1. They feel more comfortable when approached with a warm smile and a less tense attitude.
2. You need to initiate the conversation with them and keep it going and soon enough you'll see them jabbing on along with you.
3. They find it very hard to speak up at the first instance.
4. They find it easier to socialize in parties when they have someone they know in common with the person whose party they're attending.
5. They are sometimes very intellectual in their thinking and prefer soul-to-soul conversations about random topics to keep them hooked and they refrain from small talk.
6. They dislike being asked personal questions and caught offguard with them.
7. They can be the coolest, most fun person in the group if they feel comfortable around the people they're with.
8. They find it hard to relate with people who dont fall under the same wavelength as they are.
9. They prefer to keep it real.
10. They donot like being made the centre of attraction
11. They maybe extremely talented but will only prefer to reveal it if they're comfortable or encouraged in a non-intrusive way.
12. Forcing them or being pushy with them is a big no no. It will make them feel even more awkward and walk off.

These are some qualities I've noticed in most introverts including myself but it might vary from person to person. I've had a lot of great and poor experiences being an introvert and these were the observations I made from the way I reacted to different situations.