Basic script for more realistic eye movements. Eyes will snap to a set of targets.

EyeController.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class EyeController : MonoBehaviour {

	public GameObject eyeForward;
	public GameObject[] eyes;
	public GameObject mainTarget;
	public GameObject[] otherTargets;

	private float MaxAngle = 0.93f;
	private float RVal = 0.0f;

	// Update is called once per frame
	void Update () {

		if(Random.value < 0.025f)
			RVal = (Random.value-0.5f) * 0.08f;
		
		Vector3 targetPos = mainTarget.transform.position;

		// Find the best target..
		var offs = (targetPos - eyeForward.transform.position).normalized;

		float bestAng = Vector3.Dot(eyeForward.transform.forward, offs) - RVal;

		if(bestAng < MaxAngle) {

			foreach(var targ in otherTargets) {
				var offs1 = (targ.transform.position - eyeForward.transform.position).normalized;

				var newAng = Vector3.Dot(eyeForward.transform.forward, offs1);
				if(newAng > bestAng) {
					bestAng = newAng;
					targetPos = targ.transform.position;
				}
			}
		}

		// don't have eyes try to go too far..
		if(bestAng < MaxAngle) {
			targetPos = eyeForward.transform.position + eyeForward.transform.forward;
		}

		foreach(var eye in eyes) {

			var targetOffs = (targetPos - eye.transform.position).normalized;
			var rot = Quaternion.LookRotation(targetOffs, eye.transform.parent.up);
			eye.transform.rotation = Quaternion.Slerp(eye.transform.rotation, rot, 50.0f * Time.deltaTime);
		}
	}
}