These can only be played on your turn outside of Combat
When a Cast is used you must target a Soul you control that matches its requirements.
If an effect is activated that removes the targeted Soul before the Cast resolves,it will fail and be destroyed.
In addition, any effect that would change the targeted Soul and make it no longer meet the requirements of use, the Cast will also fail and be destroyed. (i.e., You activate a Cast that requires an Angel Class Soul. In response your Opponent Uses a Quick Cast to change your Souls Class. Your Soul no longer meets the requirements of the Cast, and it is destroyed.)