if you want your subject to be comfortable the best thing to do would be to distract them from the fact that they are taking a picture with whatever technique works for them. simply making them smile by cracking a joke or making a funny gesture seems to be the most common approach to catching their true emotion and making them comfortable. i think that the best way to make someone feel comfortable is to catch them in their environment, for example a little kid with a baseball bat. From that we can assume he is into baseball so we get a better picture about who he is and he feels some comfort from these items, producing a better picture as he will most likely be comfortable.