Becoming nervous in the heat of a situation. Perhaps you witnessed something not for the faint of heart. Like, how would I describe shaky legs without being so boring about it.
Well.. There are simple and less simple answers to these kinds of questions that will give you different results.
First; the simple: "He/I was nervous".. - Not a lot to misinterpret, but not a lot for the reader to feel either.
Second; the less simple: "He/I did this, said this, felt this" - the "this's" in the example above would then be replaced with examples of behaviour that people are familiar with connecting to nervousness:
- Stuttering / repeating yourself / lacking or missing words
- quick/uneasy breathing
- using or positioning your hands/arms/legs differently
- looking around quickly/without focus
- paying attention to certain details, either relevant or not
- the list goes on...
@FlyingPiMonster uses most of these in his example in his answer. You can use whichever suits your characters and the situation they're in. When you describe the qualities of an emotion/feeling/state that someone is in, the reader is more likely to feel that emotion, rather than just "know what you mean"... the what makes the character nervous could in this case be what he's paying attention to
Perhaps: Think of something that made you nervous - how did you act/react? If you cannot remember how you behave when nervous (It'd surprise me a little, but) ask someone else how they felt and or acted last time they were nervous. Perhaps even ask someone specific that your character may remind you of, if possible.
If you want to avoid being boring you avoid even alluding to the nervousness at all.
Think of some of the things that might cause your knees to shake that have nothing to do with nervousness.
Maybe your character checks the thermostat and wonders how the room can be so cold when the heat is set to 75°.
Make your character wonder if people are wondering why your character needs to piss so badly.
Your character could do something completely out of character, or give in to an old vice, like having one smoke, just this once, because it’s only once.
Have your character do something compulsive, like pull out their third stick of gum in 2 minutes, or pull their socks up repeatedly, in hopes that that will help hold them in place. Yet somehow it only makes them itch more.
If the tone is right, have your character remark that their legs are as numb as if they’d sat on the john for an hour playing Clash of Clans.
One of the most important ways you’ll establish that mythical authorial “voice” is by thinking outside the box. Anyone can convey that their character is nervous by stating that they’re sweating or that their heart is pounding. If you want to reach your readers, make it personal—personal to you, personal to your character. Bring your own experiences into the picture.
Most of the time, you won’t even need to hint at the fact that your character is nervous. That should become obvious the moment you write, "He lit up another cigarette,” because you've already established that your character gave up smoking years ago.
Spend less time describing the nervousness, and more time describing what is making your characters nervous.
Neither of them moved a muscle. Elias could hear his own heartbeat; he could even hear Jamie's nervous breaths. Two sets of footsteps were coming toward them. One was heavy and slow, like an adult's; the other seemed quicker and lighter.
(from something I'm writing)
In this short paragraph, only one sentence is devoted to actually describing nervousness: hearing one's own heartbeat and someone else's breath. Readers know from that sentence that Jamie and Elias are nervous; there is no need to tell them again.
Instead, the rest of the paragraph is about their situation. Why are they nervous? They need to avoid detection, and two mysterious figures are moving toward them. This heightens the tension in the scene and gives Elias and Jamie a reason to be nervous, which is much more effective than continuing to describe nervousness itself.
Talk to people about what happens when they get nervous so you have a strong database of ideas. And give every character a different set of reactions. The main character in the novel I'm working on likes to take leaves or flowers and slowly shred them with her fingers. Another character reacts physically sometimes to the point of violence. Others look away from the person they're talking to.
There are many ways to show nervousness and also many degrees of it. Being scared can overlap but it's different. Some people are energized by nervousness (I am...it's part of why I love to perform on stage). Others lose the ability to speak or to function or to think clearly. Some will stammer or make mistakes. Still others will behave perfectly normally except for that telltale sign only people who know them will notice.