It appears that one of the most important conditions for portrait photography is the rapport that exists between the photographer and subject/s. "Rapport" would appear to be the correct word here since it implies a level of exchange in which both parties are equal.
The photographer, it seems, needs to be in control of what is happening but not controlling. This may sound like a rather fine point but an overbearing photographer will not be able to help his subjects relax. If a person is not at ease when being photographed, he will start to get tense which is likely to show in strained musculature of the face as well as a guarded expression.
Of course, there are always exceptions to this rule and a classic example is when Yousuf Karsh photographed Sir Winston Churchill. Unable to get the expression he wanted, Karsh snatched Churchills trademark cigar from his hand and one was left with an iconic image of a glowering face, the archetypal british bulldog, who was best known for his stirring speeches to the British people during the Second World War in the mid-nineteenth century.