|Copy an Emoji|
One of the most common mistakes made by student animators is to leave out the facial expressions on their thumbnail sketches. You'd think that it would be obvious to include facial expressions, but many students don't - probably because faces can seem hard to draw.
In fact, adding rough facial expressions doesn't need to be that difficult. Think of the emotion you want to convey, and then copy an emoji. Is the character happy? Sad? Angry? There's an emoji online for every expression.