Captain Groleo

I played with Groleo several times in Joust and Melee.

When a friend of mine played Milk of the Poppy on my Groleo. I would think that since Milk of the Poppy was shown, I would use Groleo's ability first and then have Milk of the Poppy stop his ability.

Does Milk of the Poppy ability go first or Groleo's??

Short answer: I don't think you can respond to Milk of the Poppy with Captain Groleo. But I could be wrong (see above, re: "trying"), so here's my reasoning (I'm trying to learn these timing rules, so this was fun):

MARSHALLING PHASE
--> PLAYER ACTIONS (your opponent is active player, so can initiate actions to marshal cards)
1) Action Initiated: Opponent initiates action to --> marshal Milk of the Poppy
1.1) Milk of the Poppy revealed.
- the "response opportunity gate" ("an opponent ... reveals an attachment") opens for Captain Groleo's ability now, BUT you can't play responses yet.
1.2) Cost determined, valid target verified (Captain Groleo), cost paid
2) Save/Cancel Responses (responses with (1) "save" or "cancel" and (2) an open response opportunity gate)
- the only valid response here would be one that "cancels" the marshalling of Milk of the Poppy, I think
3) Action Executed: Milk of the Poppy enters play. Its constant ability has no point of initiation and takes effect immediately. Groleo's text box is blanked.
- if I'm wrong about the response opportunity gate opening in step 1.1, it's definitely open by now (still before it could be played, so it's a moot point)
4) Passive Abilities (none)
5) Responses (with an open response opportunity gate)
- Here's your first chance to step through the opportunity gate and trigger Groleo's response ability, BUT it no longer exists to be triggered, since Groleo's text box was blanked in step 3.

Eunomiac is correct on this one with a great timing chart on why. Sadly, attachments just aren't play often enough for this to be worth it.