Just to cover this properly, Choosing to resolve a card effect that has a cost (any with the black arrow) is optional, if you decline to pay the cost, you don't get the effect.
Other card abilities are not optional and must be as completely as possible when played.
It's all covered, somewhat briefly, on p9 of the rules.
So you played Insane Outcast right, but your 'or any other card' suggestion isn't correct.
Card effects are NOT optional unless they have a cost with the black arrow. On the other hand you DO NOT HAVE to play every card in your hand.