I’m not the best teacher, as I couldn’t really explain this well, I think. ^^;

But, it’s all about how much emphasis you put on certain parts of the face; more so the mouth (of course), eyebrows, and the eyes themselves.

When drawing characters in a more mad, angry state, I tend to bring the eyebrows closer together (and this depends on how close the eyebrows would even be in a more neutral position) and furrowed, while making the mouth line a bit more tight, almost as if you could imagine that character giving a huffed, “Hmpgh”. And as they talk, corners of the mouth tend to be ‘sharper’. If they’re yelling, they can loosen up a bit, but that sharpness in the corner is crucial!

With sad, I make the eyebrows stay as they probably would in neutral position. Furrowed, but in opposite format (I assume that’s how you’d describe that?). Eyes, sometimes bulged out, yet stricken with slight fear, maybe discomfort; all depends on the kind of sad!

Aah… visuals may work better for what I’m trying to get at here. :’D Here’s some actual demonstration of what I mean and extra words (with Freddy and Golden Freddy as my guinea pigs). - 

Apologies if it’s not much, but I hope it MAYBE, kinda-sorta helped a bit~