My Wish Is Your Command

If you choose a card you can’t cast (because there are no legal targets for the spell, for example), nothing happens to it. It remains in its owner’s hand.

If you cast a card “without paying its mana cost,” the value of any X in the card’s mana cost must be 0. You can’t pay any alternative costs for that card. On the other hand, if the card has optional additional costs (such as kicker or replicate), you may pay those when you cast the card. If the card has mandatory additional costs (such as Fling does), you must pay those when you cast the card.

If you choose a card, you cast it as part of the resolution of this ability. Timing restrictions based on the card’s type are ignored. Other restrictions are not (such as “Cast [this card] only during your end step”).