Walt: Are you sure?
The menacing look Walt gives to Declan and his men is enough to cause them to second-guess themselves and begin to believe Walt is capable of doing such a thing. Similar to how Gus was able to terrify Walt and Jesse in the box cutter scene without even saying a word. This little stunt also draws a dirty look from Mike, since Mike knows that the way to maintain success in this game is silently doing your job; not boasting about your accomplishments.
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