A topic from AEG's L5R CCG Rules forum, copied for Oracle of the Void from a copy of the forum saved in October 2015, so pages it didn't save are missing. Players' names are left out; the rules officials are named. Rulings are as they stood on these dates; later rulings, the rulebooks and the Rules Archive may differ.
A player, 12 April 2008
Is it legal for the opposing player to look at the 3 cards of my fate deck, then target his bowed samurai (only has bowed samurai in the battle) with my Stay Your Blade, knowing he cannot pay the cost then discard the Stay Your Blade into my Discard pile?
My contention is that it is a violation of the Good Faith Rule to do the above.
Insight - Dark Virtue Battle: Target one of your Samurai: Look at the top 3 cards of any Fate deck. If any are Action cards or Rings, you may play one of them for an additional Battle Action on it. You may rearrange the order of the remaining cards you looked at.
Stay Your Blade - Battle: Bow and move home one of your target opposed Samurai Personalities: Draw three cards.
Good Faith Rule: When an action is announced, the player must be able to target all required targets, meet all conditions of the action, and pay all mandatory costs, to the best of his or her knowledge.
Comments below the actual rule that are relevant.
If any required targets do not exist or any costs cannot be paid at this point, or if a required condition is not met, the announcement of the action is a play mistake. In that case, if the action’s ability was on an Action card, return that card to the area it came from (usually, the player's hand). The timing point after the action is announced is not reached. The action does not count against the player’s chance to take an action, nor is its ability considered used.
Brook Cunningham (rules official), 12 April 2008
Is it legal for the opposing player to look at the 3 cards of my fate deck, then target his bowed samurai (only has bowed samurai in the battle) with my Stay Your Blade, knowing he cannot pay the cost then discard the Stay Your Blade into my Discard pile?
My contention is that it is a violation of the Good Faith Rule to do the above.
Insight - Dark Virtue Battle: Target one of your Samurai: Look at the top 3 cards of any Fate deck. If any are Action cards or Rings, you may play one of them for an additional Battle Action on it. You may rearrange the order of the remaining cards you looked at.
Stay Your Blade - Battle: Bow and move home one of your target opposed Samurai Personalities: Draw three cards.
Good Faith Rule: When an action is announced, the player must be able to target all required targets, meet all conditions of the action, and pay all mandatory costs, to the best of his or her knowledge.
Comments below the actual rule that are relevant.
If any required targets do not exist or any costs cannot be paid at this point, or if a required condition is not met, the announcement of the action is a play mistake. In that case, if the action’s ability was on an Action card, return that card to the area it came from (usually, the player's hand). The timing point after the action is announced is not reached. The action does not count against the player’s chance to take an action, nor is its ability considered used.
Your contention is correct.
Insight allows an opponent to play your cards, but nothing in its text waives any other requirements or restictions, including Good Faith. He could find your Stay Your Blade, Insight's text would allow him to play it despite it being yours, but he could not announce it, knowingly unable to pay its costs, due to Good Faith.