I have a question in regards to responding to a card being played. I know that I can respond to my opponent targeting my character before his action hits it (for example: my sniper is blocking, then he plays target practice or something, and in response I snipe his atttacking character for one before sniper is destroyed). But I was wondering if my opponent plays something like flaming barbuse, and pays all the cost(including picking characters), then I choose to respond by playing something like the arcanist's Napiform item and target the Barbuse.
Would that be legally since part of the cost is picking a character and it is being played at the moment, or would I have to wait til it's in play to target the Barbuse at all? Thanks in advance for helping me with this issue.