Hello

Something occured while me and two friends were playing the other day, that i would like to get a clear answer on.

I was playing Stark, player to my left was playing Martell, and player to my right Greyjoy. There were several cards in play, but the important cards for this question is my (stark) Greatjon Umber (core: Challenges: Kneel Greatjon Umber to have him participate in the current Military or Power challenge as either the attacker or as the defender.) , and Greyjoy's Scouting Vessel ( Challenges: Kneel Scouting Vessel to choose a defending character. For the duration of the challenge, that character does not count its STR.)

Martell is first player, and once we get to challenges phase he passes. Greyjoy is now the active player. He initiates a power challenge against Martell, and kneels a character. Everyone passes player actions. Greyjoy chooses a character to stealth, and in doing this Martell can't defend the challenge.
Another player action window now opens. Martell passes, Greyjoy passes, and i activate Greyjoy Umbers ability to defend for the Martell player. Greyjoy now wishes to activate Scouting Vessel, since there now is a defending character to use it on.

My question is: can he do this, considering he already passed his turn to take any player action?

No reason why not - there are player action windows between declaring attackers and stealth and another one between declaring defenders and resolution - Player actions go round and round until everyone passes on the option to do an action

If you have actions such as strength manipulators etc. then it is to your advantage to do those as late as possible in the challenge process ... i.e. gives you a greater element of surprise over your opponent

Ok, thanks for the answer!

I didn't realise the player action went round and round. I thought once you had passed to take any player action, that was that, meaning you would have the advantage going as last player. Looks like we've been playing it wrong for a long time then.
Is the "round and round" part in the rules? Can't seem to find it. Or is it implied that that is how it works?

It's in the FAQ, but not in the Core Set rules.

Action/response options always continue clockwise in this fashion until all players consecutively pass.