More than producing a work that is preferably recognizable as the model, the challenge is to show a momentary but accurate glimpse --- seen through my eyes and expressed by my instruments --- of the whole universe making up the person I paint/draw/sculpt.
Doing this feels like about the closest I can get to that person. It is a challenging, incredibly exciting and mostly rewarding experience. For that reason I prefer to work with the flesh-and-bones person as my model, having a direct exchange rather than dealing with flatland pictures.