Basic: Realistic Eye Movement
Eyes are the most important part of human facial expression.
Eye movement and blink can deliver a very natural feedback from your character. You can also add some secondary movements to get more natural result.
At this example, I've used the Realistic Eye Movements asset from Asset Store to do that.
On the example below I've just created a small target and added as a Point Of Interest. Head, eyes and facial movements are all procedural. As this character is from Mixamo, I`ve used a built-in template.