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If a player casts a nonland card revealed with Wild Evocation’s ability, it’s put on the stack as a spell, then Wild Evocation’s ability finishes resolving. The spell will then resolve as normal, after players get a chance to cast spells and activate abilities.
If, as the result of Wild Evocation’s ability, a player casts a card “without paying its mana cost,” any X in the mana cost will be 0, and that player can’t pay any alternative costs (such as Demon of Death’s Gate’s alternative cost). On the other hand, if the card has optional additional costs (such as kicker or multikicker), the player may pay those.
If a player reveals a nonland card, that player must cast that card if it’s possible to do so, even if they don’t want to. The player casts it as part of the resolution of Wild Evocation’s ability. Timing restrictions based on the card’s type (such as creature or sorcery) are ignored. Other play restrictions are not (such as “Cast [this card] only during combat”).
If casting the revealed card involves paying a mandatory additional cost (such as the one Fling has), the player casting that card must pay that cost if able. If they can’t, the card remains uncast in their hand. If the mandatory additional cost includes a mana payment, the situation is more complex. If the player has enough mana in their mana pool to pay the cost, that player must do so. If the player can’t possibly pay the cost, the card remains uncast in their hand. However, if the player has the means to produce enough mana to pay the cost, then they have a choice: The player may cast the card, produce mana, and pay the cost. Or the player may choose to activate no mana abilities, thus making the card impossible to cast because the additional mana can’t be paid.
If a player reveals a nonland card, but can’t cast it due to a lack of legal targets or a play restriction, it simply remains in their hand.