Casey & Raph, Hotheads

If a spell has {X} in its mana cost, you must choose 0 as the value of X when playing it without paying its mana cost.

The target of the first mode must follow all timing rules for cards played this way. For example, if the exiled card is a land card, they may play it only during their main phase while the stack is empty and only if they have an available land play remaining.

If you cast a spell "without paying its mana cost," you can't choose to cast it for any alternative costs. You can, however, pay additional costs. If the spell has any mandatory additional costs, those must be paid to cast the spell.