A lot of the time you will find people are often afraid of having their photograph taken which will come across in their expression and posture in the images.
Our job as photographers is to make sure we really connected with the people we are photographing, making them feel comfortable and helping them to build up their confidence so we can break down that nervous tension they might be feeling.
Ask them what their interest are, such as what music they like, what are their passions! also stop and show them what you are shooting once they see how good the images are it will help put them at ease with the image making process.
Once they are relaxed and comfortable with you the process is much more enjoyable for the two of you and that way you will get the best results in final images, here are a few portraits i have taken.