PlayerMovement.cs

using UnityEngine;

public class PlayerMovement : MonoBehaviour {

    public Camera playerCamera = null;
    public Animator animator = null;

    public float moveSpeed = 5.0f;
    public float crouchedMoveSpeed = 2.5f;
    public float rotationSpeed = 5.0f;

    private bool crouched = false;

    private float speedZ = 0.0f;

    void Update () {

        float moveH = Input.GetAxis ("Horizontal");
        float moveV = Input.GetAxis ("Vertical");

        bool toggleCrouch = Input.GetButtonDown ("Toggle crouch");

        Move (moveH, moveV, toggleCrouch);
        Animate ();
    }

    private void Move (float moveH, float moveV, bool toggleCrouch) {

        if (toggleCrouch) {
            crouched = !crouched;
        }

        float speed = crouched ? crouchedMoveSpeed : moveSpeed;

        Vector3 movement = new Vector3 (moveH, 0.0f, moveV).normalized;
        transform.rotation = Rotation (movement);

        speedZ = movement.magnitude;
        transform.Translate (new Vector3 (0, 0, speedZ * speed * Time.deltaTime));
    }

    private Quaternion Rotation (Vector3 movement) {

        Vector3 lookAt = movement.z > 0.8f ?
            new Vector3 (playerCamera.transform.forward.x, 0, playerCamera.transform.forward.z) :
            Vector3.zero;
        Vector3 forward = Vector3.RotateTowards (transform.forward, lookAt + movement, rotationSpeed * Time.deltaTime, 0.0f);
        return Quaternion.LookRotation (forward);
    }

    private void Animate () {

        animator.SetFloat ("Vertical", speedZ);
    }

}