There's no challenge in explaining how something happened. Anyone can tell you about their day. Anyone can tell you about someone else's day. As writers, what we do is not explain how something happened; what we do is tell a story. And part of telling that story is carrying our readers along with suspenseful, entrancing words. "She was nervous," isn't suspenseful or entrancing, and while it gets the message across (let's face: when you read "She was nervous," you can't help bu think that she was nervous) it doesn't really pull the reader into what you're saying. It doesn't give them a clear picture of what exactly your character is going through.
So how do you pull out your character's feelings to make the reader feel with them? I have a few tips:
More words isn't a bad thing.
As long as you're not droning on about something, it's far from a bad thing to go into a somewhat lengthy description of what your character is doing. Tell us what shade of red the color of their cheeks are turning. Explain that their hand is placed over their mouth. Talk about the creases at the corners of their eyes when they smile. I've never been able to express enough to people that more description in their work can do WONDERS!
Put yourself in your character's shoes.
Sometimes imagining how a character will react physically to a situation isn't an easy task. You need to experience the emotion yourself. For just a moment, allow yourself to become your character; to take on their mind and their quirks and their physicality. Imagine that what's happening in the story is happening to you. How do you react? Even if you can't put yourself completely into your character's mind, your own knowledge of how you were react will give you a great starting point.
Don't just explain one character's reactions.
The reactions of those surrounding your character (assuming they're not alone) can be as useful a tool as explaining the character's reactions. It's especially useful at the beginning of a chapter, where you can use a "zoom in," in which you start out explaining the general feeling of the room, zoom into explaining some of the characters and then zoom into the main character(s). Showing the reader how the others feel about the situation will help to imbue some feelings into them.
Words (or the lack thereof) can be expressive.
I'm sure this doesn't need much saying. But a character's verbal reactions to a situation can be extremely useful, when used alongside a physical reaction, even when that verbal reaction is non-existent. A character's silence can give off a lot of different emotions. When partnered with a glare, it gives off a "Don't piss me off!" sort of feel. When partnered with a blank expression, it says "I'm in deep thought and can't be bothered to respond." And when partnered with a smile, well, they're just too happy for words, aren't they?
While explaining an event isn't a bad thing, it's not always the right thing. Explaining a characters physical reactions is sometimes a better use of words than saying how they react.
AND NOW I HAVE A CHALLENGE FOR YOU!
Do a Google search and fine two different pictures of a person's face (it can be two different people). Describe each one of the expressions without using words that describe their mood (eg. "nervous," "angry," "doubtful," etc.).