Can Nocturne be played directly to a battlefield during a showdown when revealed by an effect like Teemo's ability?

Ruling: Yes, Nocturne can be played immediately to a battlefield you control during a showdown when his triggered ability activates from being revealed/looked at, even though he doesn't have the Action or Reaction keyword.

Nuances:

• The interaction specifically with Teemo might change pending future clarification (there's discussion about whether "look" counts as "reveal" for Nocturne's trigger)
• The general rule is you can only play units to battlefields during showdown if they have Action/Reaction keywords OR if an effect allows you to play them
• Nocturne falls into the second category - his triggered ability allows immediate play
• You must control the battlefield to play a unit there during showdown