An interesting scenario occurred to me and I wanted to make sure my reading of it was correct.

Stark player has Lucas Blackwood, Grey Wind (attachment version), a bunch of other characters and has totally cleared their opponent's board. Things are looking good for him. Stark's going first, marshals some stuff, passes active player to his opponent. His opponent marshals some powerful character down that Stark would love to kill while there's no claim soak up. He kills poor Lucas with Grey Wind, initiates his Military Challenge, and here's my big question.

Can his opponent use their actions within the framework of declaring a challenge to marshal characters?

My take is yes, but I'm curious to see if there's something I missed.

In any case, the opponent will get an action before the Stark player can initiate the challenge, because here's the timing:

• Stark's action: use Grey Wind to kill Lucas Blackwood, respond with Lucas Blackwood
• Opponent's action
• Stark's next action: initiate the military challenge granted by Lucas Blackwood's response

Now the challenge has initiated, does it change the fact that it is still the Marshalling phase with Stark's target the active player? Like you, I'd say no. During the newly opened action window, players may still take any action they are normally allowed to take and no other (Ghaston Grey is no more usable in this situation than during an epic phase, for instance). And even if the Stark player has declared Brienne of Tarth (PotS) as an attacker, the active player may still marshall cards, because marshalling a card is not a triggered effect.

So the way to go for our violent Stark friend is to use Grey Wind when his target is not the active player (preferably while he is).

If I'm wrong, may someone more knowledgeable correct me.

Think your bang on the money Khudzlin, as long as starks target is the active player he could play characters out, even during the challenge.