How to draw Knuckles' Emotions

Anger
This is probably the emotion we see Knuckles displaying the most, because he's more inclined to think with his fists than his head. Here are a few examples of various angry moods.
 
We'll start with the sullen look. Knuckles' eyebrows draw down over his eyes, and his face is down. Notice his nose. The nose always points in the direction the character is looking. When Knuckles frowns, draw a line along the bridge of his nose. It makes his expression more pronounced. See the Misc. section for more details on echidna eyebrows.
Suspicion. When someone looks suspicious, their eyes seldom look ahead--they tend to look sideways, as if they're thinking off to the side somewhere. Knuckles' eyebrows draw far down over his eyes--one of the ways to make him look "echidna" is to give him enormous eyebrows and small eyes--and his cheek comes up a bit. His mouth is placed rather high, too. His dreadlocks always hang over his cheeks, so make sure you include that when he's sitting still.
Fuming. Posture says a lot with Knuckles' emotions. At this point he's standing still, so chances are he's not going to inflict beatdown on anyone yet. Notice how square the corners of his eyes are, and notice the tilt of his head (watch the nose!). When a character is angry they hold their head down, and their eyes nearly disappear beneath their eyebrows. At this point, Knuckles is showing some teeth. Vertical teeth usually imply anger or cunning, while horizontal teeth are associated with smiles. Horizontal teeth, however, can also be made into snarls.
Ticked off. Knux is moving forward and showing horizontal, jagged teeth (a snarl). At this point it'd be wise to get out of his way. Notice how his brows are drawn together in the middle, forming a little bulge. The thicker and darker the bulge, the angrier Knux looks. Make his eyes slanty, like Metal Sonic's. Slanty eyes imply ferocity. Again, his head is down. See how much of the top of his head is visible? 
Angry people also carry their arms at their sides with their fists clenched.
Attack! Head down, eyes squinted, teeth bared to the roots. The flying dreadlocks say it all, as do the "swoosh" lines tracing his arm. 
Note: Knuckles' dreadlocks will always move in the opposite direction that Knuckles is moving.

Surprise
This doesn't happen much, but it's good to know how to handle when it does.


 Surprise. To convey this emotion, first look at his nose. It's pointed up. In fact, his whole head is slanted back, showing more of his muzzle and less of the top of his head. His eyes are rounded, and his mouth is placed rather low, and showing a tongue instead of teeth. Notice that his hands are up and open. When someone is startled, the reaction is to shield their face. This motion is echoed when a character is surprised: their hands come up a bit, and they are never clenched.
Shock. This is a bigger emotion than surprise. Knux's eyes are even rounder, his muzzle is even higher, and there are little "sunshine" rays coming off the top of his head to indicate that he is startled. Notice his eyebrows don't draw together in the middle at all, and his dreadlocks tend to swing around a lot. With cartoon characters, shock tends to stretch their heads vertically. The more exaggerated this is, the more shocked the character.

Other Emotions
Notes on other common expressions.


 Cringe. When someone cringes, they frown and close their eyes at the same time. Knuckles' eyebrows draw down, but not as harshly as if he were angry, and give him two "closed eyelid" lines to indicate a squint. Make him frown, too. Nobody ever cringes over anything happy.
Sadness/sympathy. Sadness is explained in the Misc. section. Sympathy, however, is not so pronounced as sadness. Knuckles' eyebrows are drawn forward, but his eyes are not half-closed.
The Evil Idea look. The echidna got a wonderful, awful idea! One eyebrow is radically lowered while the other is radically lifted, and the smile is jagged. His brows draw together a little, and his visible eye will be half-closed. To give the impression of "thinking", bring his hand up to his mouth in the classic "thoughtful" gesture.
Worry. Knux's eyebrows will be drawn forward just a little, and his mouth will be a flat line. Draw his dreadlocks a little further over his face than usual, and have his cheek come up against his eye. The poor guy has something serious on his mind.