This links back into the well-trod and oft-recited ‘show don’t tell’ rule. Characters need to display emotion. That’s a core part of what drags your reader in and helps immersion. How are readers supposed to attach themselves to a cardboard cut-out?
To convey emotion, you need to put yourself into your character’s shoes and show your readers. Simply saying ‘George was sad’ doesn’t give your reader much insight. How did you know he was sad? Was it the tears streaming down his face? The quivering bottom lip? That’s what you need to put in, show your readers, and use to help them look through the character’s eyes.
Dialogue is also very important and useful when it comes to conveying emotion. A slight change in word choice can drastically alter the tone and impact. Someone who’s angry will speak differently than someone who’s sobbing or in shock. While characters do have a voice and thus a particular way of speaking, there are still changes between emotions. A more intense, negative emotion such as sadness or a state of shock can lead to far more pauses and rambling. Elation, on the other hand, can lead to lots of exclamations and bouncing around the topics at hand.
When conveying emotion, you should think about the huge array of cues we take for granted in our everyday life. Of course, the reader will understand if you put it very simply, but it takes away some depth and removes something from the experience.
In summary, help your reader see things through your character’s eyes. Don’t tell them that the characters are sad or happy, show them. Describe the unspoken cues which we use in real life, the body language and facial expressions. Don’t forget the dialogue, too.