Pattern for reusable Unity Components using interfaces.

DemoAirHorn

public class DemoAirHorn : MonoBehaviour, IThingBehaviour
{
    public void DoAThing()
    {
        Debug.Log("EEEEEEEEEEEEEEEEEEEEEEEEERRRRRRRRRRRRRRRRRRRRRRRRRRRRRRR");
    }
    
    public int ReturnAThing(int number)
    {
        return number + 1;
    }
}

DemoPlayer

public class DemoPlayer : MonoBehaviour, IThingBehaviour
{
    public void DoAThing()
    {
        Debug.Log("I, a player, have done a thing.");
    }
    
    public int ReturnAThing(int number)
    {
        return 10 * number;
    }
}

IThingBehaviour

public interface IThingBehaviour
{
    void DoAThing();
    
    int ReturnAThing(int number);
}

ThingComponent

using UnityEngine;

public class ThingComponent : MonoBehaviour
{
    private IThingBehaviour _thingBehaviour = null;

    //On awake, the component finds a component on the object with the required interface.
    private void Awake()
    {
        _thingBehaviour = GetComponent<IThingBehaviour>();
        if(_thingBehaviour == null)
        {
            Debug.Log($"{gameObject.name} is missing a MonoBehaviour that implements IThingBehaviour!");
        }
    }


    //Example of internal component logic that triggers method on the interface implementation component.
    private void Update()
    {
        if(Input.GetKeyDown(KeyCode.Space))
        {
             _thingBehaviour.DoAThing();
        }
        
        if(Input.GetKeyDown(KeyCode.Enter))
        {
             var number = _thingBehaviour.ReturnAThing(12);
             Debug.Log(number);
        }
    }
}

A way in Unity to make components you can add to a GameObject and only need to couple the GameObject's main script to them. The components don't need to know anything about the object they're attached to, only that they have a component that implements the interface. To add the component to a new object, you just add it in the inspector and implement the interface on a component on your GameObject.

The workflow for this would be:

1. Add ThingComponent to the GameObject.
2. Add the IThingBehaviour interface to a new or existing script on your object.
3. Implement the interface with your object specific implementation.