Where the Reset Method only gets called once when the component is first added, Unity will call OnValidate much more often:

• When the component is first added or the Reset function is used from its context menu (same as Reset).
• When the component is loaded, such as when opening a scene, editing a prefab, or after you modify a script and Unity recompiles it.
• When the component is modified in the Inspector.

So we can use it to simply get the required component if one wasn't already assigned:

using UnityEngine;

public sealed class OnValidateMethodExample : MonoBehaviour
{
    [SerializeField] private Rigidbody _Rigidbody;

    private void OnValidate()
    {
        if (_Rigidbody == null)
            TryGetComponent(out _Rigidbody);
    }

    private void FixedUpdate()
    {
        _Rigidbody.AddForce(Vector3.up * 10);
    }
}

Advantages

• All the same advantages of the Reset Method, which are:
  • Fastest runtime performance - Unity deserializing the reference to set the field is faster than a script calling GetComponent on startup and the cost of calling it in Edit Mode doesn't actually matter.
  • The Inspector shows what other components the OnValidateMethodExample depends on and which one is actually being used.
  • The other components can be anywhere. The script isn't hard-coded with the assumption that the Rigidbody will be attached to the same GameObject.
  • It will automatically find the required Rigidbody (if there is one) when you first add the OnValidateMethodExample component.
• If you add or rename a serialized field after you have already added the script to an object, the OnValidate method will get called again.

Disadvantages