...how can anyone be smacked inside the face?
Hi again. With questions of idiom there is not necessarily a simple and logical answer. It often just feels right to the native speaker.
For example: If I say that I could give someone a "slap on/in the face", with both forms sounding natural to me, I would nevertheless invariably say "slap on the cheek". But this doesn't mean that referring to a very specific part of the body is a reliable indicator, because I would also tend to say "a kick in the knee", rather than "on the knee".
Where there is a very aggressive tone, "in the face" becomes more common.
"If you lie to me you'll get a smack in the face".
"I'll smack you in the face if you touch my girl".
For both those examples "on the face" would have less force.
So it's difficult to give guidelines except to suggest that you note the context in which you hear the phrases, and also that there is often more than one acceptable form.
