Most people have no experience of posing for a portrait. In a commissioned portrait, it is important that the model shows off his best side. He or she should give me something that I can give back in the painting. Usually I am able to get a satisfactory photograph of the person. (As you know, for commissioned portraits I use photographic sources.) Sometimes I take more than fifty shots, other times I take only ten pictures, certain that the pictures will be useful. A good relation with the sitter is one of the most important ingredients. To achieve that I am sometimes a bit of an actor.
Very occasionally it happens that a model cannot relax and remains formal during the session, whatever I try. Managing to convey that his performance is a disaster is impossible.
I have a solution that often works. I do multiple poses and halfway through I say that I'm excited, and that it's going great. This falsehood will bring life to the model. Sweet lies that I know I can rely on in case of an emergency.