When painting busts the face is essential. With color variations and vibrancy at the right spots and natural detail work you can make it alive. Therefore observation of the real thing is unavoidable.
Additional to above is that you are emotional connected to what you paint. Think about the story of the character you paint. Imagine what he went through, where he is right now, what is ahead of the character. Feel his story that you roll out in your imagination and painting a characterful face will just flow.
The size of this face is like a fingernail from a thumb.