Question regarding Ambush
For reference, Ambush is defined in rulebook p. 3. The applicable part to answer your question is "A player may, as a player action during the challenges phase,..." So, it's an action, and has the same timing rules as any other action. If we then switch to p. 25, where it defines the challenges phase, there are 3 action windows during that phase. The first occurs before the attacker declares a challenge, then another one after he's declared it, and a third after the defender chooses defenses.
So, to your specifics. Yes, as the active player you may ambush, pay X, and put a character into play. If you want that character to participate in the challenge, you would need to do that before declaring the challenge. You can also do that during your opponent's turn. For the defender, the second action window - after the attacker has declared who's involved in the challenges - is your best bet. In the third window, it's allowed, but pointless as the character goes into play, but both sides have already committed to who's involved in the challenges (pending future cards that might allow you to add someone into a challenge midstream, but at this point that can't be done.)
Hope that answers your questions.
Agarett is correct. There are several places that you can ambush a character in. The only place I disagree is that the window after characters have been committed is "pointless". While certainly not quite as useful, at the moment, perhaps, it can still be used to add strength that your opponent didn't account for for winning dominance. It would be possible to throw in that ambush character on the final challenge of the phase after attackers and defenders are declared and ****** the extra power away from your opponent who thinks he will have more standing strength/gold than you in dominance. Like I said... perhaps not the most useful way of using ambush, but I don't think I would call that window "pointless".
Or, you know, Ambush in the character after declaring defenders in a MIL challenge to use for claim. Very useful if all you other characters have power on them, or are needed for other purposes.
Thanks for the help guys!
I actually read that section on page 3 of the Rules Reference. Somehow "as a player action" didn't click in my head as Ambush="Challenge Action: pay X to put this card into play from your hand".
My bad--sorry for the obvious question!
Also Areo Hotah can drastically affect the outcome of a challenge when ambushed in prior to the challenge resolution.
I Ambushed in a Burned Men in an Intrigue challenge just to keep my opponent from discarding it.