Focusing on proportions was what lead to my prior success as a portrait artist.
One of the major differences between realism, semi realism, and caricature lies within the variants of certain facial features in relation to each other. for example considering the size of eye (specifically in relation to the nose)
The more those relationships are emphasized, the more cartoony your impressions will appear.
So in contrast, the mores subtle these variants are (provided they are consistent) the more realistic of an impression you will get.
Since proportions tend to require a lot of adjustments, many self-taught artists tend to over-look this fundamental principal of design. With that said... if you happen to have strong grasp of proportion drawing, it may be worth considering inconsistent rendering of the mouth and nose in relation to the eye which is another common occurrence. Noses tend to be drawn with fewer shapes defined and mouths having too many rendering lines.
The reoccuring theme usually comes down to an overwhelming contrast in how facial features are presented in relationship to each other.