Specifically here

There's not a lot to say that isn't already in the tutorial, but to summarize part of it and expand on another, expressions tend to be whole face things. In art you never want to go subtle on things, be dramatic. Like, encyclopedia dramatica dramatic. Happy (and painful) expressions tend to wring the face up vertically. Sad expressions tend to kind of melt the face and everything seems to droop. Anger pulls almost pulls the face inward horizontally as you scowl, but will squash and stretch a bit vertically as the expression becomes more intense (eyes open more, but the brow furrows and the cheeks may or may not bunch up to create a shelf for the lower eyelid). The more intense the rage, the harder the lines become on a face. Really the more genuine and intense any expression, the more the lines of the face are exaggerated toward a particular end of the spectrum. Happy or sad expressions will have rounder lines, basically the lighter or more oppressive moods. Intense expressions will have hard lines, including panic, fear, and anger.

My favourite part of the comic/tutorial is "Watch people. Watch them hard." It's true, and comical if you actually take it to this extreme. I've done it a few times with friends when they've been especially maudlin or grumpy and the way their expressions morph are priceless. But seriously, invasion of space is not cool and should only be done with people you know at least on a regular basis. Please, observe responsibly. </PSA>

The part that isn't quite elabourated on but is mentioned is animals have expressions unique to them. At one point down the long collage of expressions, Viktor (the eyepatch wearing surly guy who seems to be in a permanent state of grump) wrings up his face in a manner that felines are known to do, with tense muscles on the top of the muzzle making very visible lines, as his jowls arc and reveal sharp fangs. Still, his face has a humanity to it, mostly in how the brow line shapes and the fact that he's got a hunched look to him. In the next picture below he ceases any human expression and has instead a very feral look of aggression that draws on his feline half, like he's attacking something.

Also common to all the Lackadaisy cats, every ear emotes with the face. Abuse them! Only the floppiest of ears will fail to convey vast emotions, and even then those floppy ears will perk up when their owner is excited or happy (shifting a bit further up the head toward the top. Not much, but it's noticeable). Perturbed, angry, sad, expressions are often joined to ears pinned back against the head, or at least off at odd angles. Happy or alert expressions often see the ears front and center, maximum perkiness as they face the thing their owner is interested in.

Expression isn't just a whole face thing (it is most definitely a full face thing at the least), it's a whole body thing too. Shoulders can convey any number of emotions based on the face they're attached to: Defensiveness while being scolded, a readiness to fight when enraged, and tension when faking a smile because they know something that the person they're talking with doesn't and it's bad in some way can all be amplified by the shoulders being bunched up in a kind of shrug. Meanwhile hunched, heavily sloped shoulders can show dejection, defeat (particularly to a command from someone who's barely your superior in a hierarchy), hopelessness, consignment ("ugh, fine, I'll carry your bags", after you realize you're going to do what your girlfriend wants anyway), etc. And the whole of the expression can radiate down through the posture of the character as well. Basically, look to Disney for all of that, because they've been doing it for decades.