Secret = Canon Statement no matter what the contents are.

If Lisa reveals the secrete herself i.e. by telling another player, she losses credibility.
If a player, say Reed reveals the secrete by figuring it out, he losses credibility.

If Reed confronts Lisa about the secrete and Lisa admits it, they both lose credibility.
If Reed confronts Lisa about the secrete and Lisa denies it, only Reed lose credibility.

How does a player figure what the secrete is? A player must develop the plot in such a way that whatever the secrete Lisa is hiding could be figured out by digging for evidences regarding the matter. Unlike Lisa who may reveal the secrete anytime.

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I hope the clears up a few things Silent.