how to typecast from GameObject

for that kind of behavior you're actually adding components to game objects. you don't derive from game object. if you want to check whether a game object has a specific component, use something like:

 if (GetComponent<EquippableItem>() != null)
 {
 }

but just a reminder, in unity, you don't actually need to know the type of the object/component or anything actually. just send a message whenever you want something to happen. then handle that event in the class itself. in your case:

 class EquippableItem : MonoBehaviour
 {
    void OnMyEvent()
    {
      DoSomethingHere();
    }
 }

then whenever you want to trigger that just call

 targetGameObject.SendMessage("OnMyEvent")

for the game object we're talking about.

take a look at these methods from component:

http://unity3d.com/support/documentation/ScriptReference/Component.SendMessage.html http://unity3d.com/support/documentation/ScriptReference/Component.BroadcastMessage.html http://unity3d.com/support/documentation/ScriptReference/Component.SendMessageUpwards.html

also for accessing components:

http://unity3d.com/support/documentation/ScriptReference/Component.GetComponent.html http://unity3d.com/support/documentation/ScriptReference/Component.GetComponentInChildren.html