Can I just confirm that using Ambush is a player action

My opponent wanted to ambush a character into play after resolution but before the claim was paid ... but there is no action window

Also, if a defender declares no defenders and someone wants to step in a support, I take it there is no action window before the supporter's defenders are declared

thanks

I just saw, the original question was double posted, you can view the answers here:

http://community.fantasyflightgames.com/index.php?/topic/107768-ambush/

I'll leave my answer in just cause

Yes, ambush is a player action. From the core rules:

You may put a card with the “Ambush” keyword into play from your hand as an “Any Phase:” action, by paying its printed gold cost with influence.

You are also correct in that there is no player action window between "Determining winner of challenge" and "Challenge result is implemented". So he would not be able to ambush in a character in that window. The closest he could do it was after defenders were declared before the winner was determined.