Vector2 vec = new Vector2(horizontal, vertical);
vec = Vector2.ClampMagnitude(vec, 1);

Vector3 camF = cam.transform.forward;
Vector3 camR = cam.transform.right;

camF.y = 0;
camR.y = 0;
camF = camF.normalized;
camR = camR.normalized;

transform.position += (camF * vec.y + camR * vec.x) * Time.deltaTime * MoveSpeed;

Vector3 deltaPosition = transform.position - prevPosition;

if (deltaPosition != Vector3.zero)
{
    transform.forward = deltaPosition;
}

prevPosition = transform.position;