Emotions in a novel are fundamental, necessary things in a story. If your main character doesn't exhibit emotions, the reader won't be able to relate or connect as well to him/her. It will be as exciting as reading about a paper doll cutout--flat and two-dimensional.
On the other hand, it is entirely possible to overdo it. You don't want so many emotions on the page that it becomes exhausting to read. Sprinkle them judiciously, because if your character is emoting intensely about EVERY event he/she encounters, it will dilute the intensity of your major climactic scenes.
You don't have to record every palpitating heartbeat or hand tremble; sometimes you should pare back, and imply emotion rather than directly describe it (through dialogue, action, or other means). Especially in dialogue, describing emotions during a scene can bog down the conversation flow.
Major Emoting EXAMPLE:
Brendan slid his Geometry book out and closed his locker with a gentle snap. He winced as someone down the row slammed theirs, setting off a domino effect of over-exuberant crashes, rattles, and bangs from the rest of the sophomore lockers. He trudged to class, his dread piling up like a mound of trash shoved into his path by a junkyard bulldozer. As he entered and his eyes shot to the figure in the second seat, fourth row. His mouth went dry, his saliva sucked into oblivion in microseconds.
Mikaela Thompson. Gorgeous, driven to succeed, and deadly.
He slid into his seat behind hers. She didn't even bother to turn around.
"Did you finish?" she said, her voice needling him like pins plunged into a voodoo doll made in his likeness.
"Uh, sorta." He tried to swallow. With no saliva, it was similar to the dry heaves of hurling without anything in his stomach.
She half-turned, showing the sheer beauty of her profile and sending his heart into a different kind of racing tempo. "What does 'sorta' mean, Brendy? Either you finished the homework for me, or you didn't."
He slipped the sheet of paper from his Geometry book and held it out, the page rattling to the tempo of his shaking fingers. "I had trouble with a few of them. Not sure if they're right."
"That's unacceptable," she hissed, the unseen snakes of her voice showing jagged fangs, poised to bite.
"I did my best." The paper continued to rattle in mid-air. His mental self threw himself at her feet, begging, pleading, groveling upon her fifty-dollar shoes. "I'm sure you'll at least get a B+ on it."
"That's not good enough," she said with a snarl, ripping the page from his fingers so fast he flinched from a wicked paper cut. "I'm just going to have to find someone else to help me out."
As she turned her back on him, Brendan's world exploded into worthless shrapnel. His head sank to his desk with a hollow thud, and his brains and emotions dissolved into screaming, spinning nanoparticles. The cut on his finger oozed red in a garish announcement of his failure.
While some of these descriptions may be interesting, there are probably waaaaaay too many of them. Sometimes, less is more.
What do you think?
Keep some of these expressions of emotions, and slash others? Fine as is?
Can you think of other things about this passage that would improve it?