If you are finding it difficult to get a genuine smile captured, here’s what you can do.
Focus on an area of the face in advance.
Then, ask the subject to smile a little
Now that you have the correct focus, you should get the smile right. However, if the subject doesn’t like the smile, you will have to do something smart; just try to be creative and do something to make the subject smile genuinely. At this stage, it is completely OK to say something unexpected (and not offensive), to make the subject smile. Thanks to the focusing you have already done, you might be able to capture a genuine smile by now.
Before you start the photographing session, it is always important to be friendlier with the subject. Make him or her feel relaxed. The more comfortable the subject is the more genuine the smile will be. So, just have a casual conversation with the subject before start photographing.