Tilt/Rotation based on direction of movement? (player follows mouse hover)

I currently have the following code, which makes the player follow the mouse hover. (player always under mouse)

Before, I was using arrow keys. The line at the bottom made the player tilt in the direction of movement.

However, this tilt rotation doesn't work with mouse hover. I would like the player to tilt/rotate in the direction of movement. Any tips for a beginner?

Thanks!

 using UnityEngine;
 using System.Collections;
 
 public class FollowMouse : MonoBehaviour {
 
     public float distance = 1.0f;
     public float tilt;
 
     // Use this for initialization
     void Start () {
 
     }
 
     // Update is called once per frame
     void Update () {
         Vector3 mousePosition = Input.mousePosition;
         mousePosition.z = distance;
        transform.position = Camera.main.ScreenToWorldPoint(mousePosition);
     }
     void FixedUpdate()
     {
         GetComponent<Rigidbody> ().rotation = Quaternion.Euler (0.0f, 0.0f, GetComponent<Rigidbody> ().velocity.x * -tilt);
     }
 }