I noticed that there all kind of facial expressions explained that you can apply to your character but I didn’t found a nervous one. So here are my two cents.
An easy way to express that your character is nervous is to let her chew on her lower lip. Notice that the left side is just drawn a little thinner and some folds are indicated. A part of the lip seem to be hidden behind the teeth if you draw it this way. Looks a little nervous, doesn’t it?
To let her look a little more nervous you just have to draw this side of the lip a little thinner. Looks a little more nervous, he?
You can make the inked lines a little more neat than I did in this fast drawn examples. I suggest you experiment a little with it. The only think to keep in mind is that one side of the lip is hidden behind the teeth. Try some more extreme variants and have fun.
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