Did you show interest in his idea? Please tell me you showed interest in his idea.
If you didn't show interest, they'll assume you're in cahoots with his colleagues and that you dismissed the idea because you hoped to make the man doubt himself and throw him off the trail.
I guess you're new to this, but it would be a shame if you got pulled into this unprepared just because you didn't know the right way to respond in these situations. If it is too late, I can offer some small help, but you don't know who I am or where I'm from, alright? I'm just some person on the internet and you don't have information to share about me. Practice that exact sentence until you've got it memorized.
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Ace Detective in Sir Chris' Police
http://i56.tinypic.com/2enwxo0.jpg