Common ways to show emotion:
Physical Action (beats): gestures, movement, ticks & tells that express emotion
Internal Sensations: bodily reaction known only to the POV character
Thoughts: reactive & emotionally charged thoughts caused by stimulus
Dialogue: revealing emotion verbally (and sometimes showing by what is not said!)
POV Narrative: internal musings/reflection delivered by a POV character toward a situation or setting
A balance of these elements creates a satisfying window into the character's emotional state, but too much causes an overload of sensory information. It slows the pace, creates melodrama and disrupts the reader's belief in both the character and the events unfolding.
Over-expressing occurs when we try too hard to reinforce an emotional state to the reader. Here's an example of how this can happen. First, we need an emotion. Let's go with GUILT.
Mrs Henderson lifted her day planner and rifled through the papers on her desk. "I don't understand--the stapler was here right before lunch. Did someone use it and forget to put it back?"
Amanda slid down in her seat, heat burning through her. Stupid! Why did I take it?
A very simple situation--not a lot is needed to get into Amanda's emotional state, right? Internal and external cues work together.
So what if I did this instead:
Amanda fumbled her library book open and shoved her nose deep into the pages so she wouldn't have to look at the teacher.
Okay, again, this works.
Amanda shifted in her seat, grazing her knee on the bottom of her desk. What if the teacher knew? What if she asked everyone to pull out their desk trays?
Yep, still showing guilt, blending external cues and thoughts, which gives her guilt a paranoid edge.
Now...what if we put it all together?
Amanda fumbled her library book open and shoved her nose deep into the pages so she wouldn't have to look at the teacher. Shifting about, she slid down in her seat and her knee grazed the bottom of her desk where she'd hid the stapler. Heat burned through her. What if the teacher knew? What if she asked everyone to pull out their desk trays? Stupid! Why did I take it?
WAAAY too much showing for this simple scenario and a medium level emotion, isn't it? Can you imagine if I'd chosen a situation rife with stronger emotions, like a character running for their life or witnessing a murder?
The trick to finding a good balance of emotional showing is remembering that Readers are smart. They will pick up on the emotion without it being hammered into them. A few strong bits of showing is almost always better than a weighty clump of detail.
Like all areas of writing, this gets easier with PRACTICE. As you hone your Writer's Intuition, you become a better judge of just how many cues are needed to get the character's emotion across. Trust in your showing skills!