Sneak and Show wants to resolve Show and Tell or Sneak Attack to cheat in Emrakul, the Aeons Torn, Griselbrand, or Atraxa, Grand Unifier, so our sideboard plan is built entirely around denying that cheat. Containment Priest is the premier answer since any nontoken creature put onto the battlefield without being cast (via Show and Tell or Sneak Attack) gets exiled instead, completely shutting down their core gameplan while they're forced to actually cast their giant threats. Deafening Silence also slows them down since Show and Tell, Sneak Attack, and their cantrips are all noncreature spells they need to chain together. Flickerwisp and Pre-War Formalwear are too slow and don't interact with their combo pieces, so they come out.