Beware of "cheesy" photographic smiles. They have rarely been and are not a recommended feature for most painted portraits, (unless the natural "being" of the sitter is to smile.) More often than not, a constant toothy grin, bearing down on the viewer from a painted portrait, can become annoying after a time and is more reminiscent of a poster-sized snapshot than a painted portrait.
A good portrait rarely resembles a family snapshot which says little if anything about the person photographed. A good portrait tells a story both physical and spiritual about the person represented.