"ISTps normally have a very characteristic passionless facial expression, indeed their whole appearance shows a lack of emotion which may be interpreted as calmness, mystery or inaccessibility. Some ISTps hide their lack of emotion behind the slightly artificial smile of a person who is seeking sensible pleasures. Their facial expressions often show scepticism or mistrust."
"Their mouth line is often calm, straight and turned slightly downwards at the corners. ISTps have a characteristic one sided smile which when combined with their scepticism can sometimes be interpreted as self-satisfaction. Their teeth are often of equal width and their mouth is usually kept tightly shut even when relaxed."
(from a socionics profile)