We had some doubts the other day, whether or not you can respond to a development being played from the hand.

Can I, in responce to the development being played, trigger (and resolve) an action before the development is actually in play? Or is it in play the moment it physically hits the table, since people don't declare "I want to play a development" before they put it on the table.

Surely you can. Playing a development from your hand is a action that can not just be played in response to anything. However, you can respond to the action, playing a development from your hand.

What is more, some actions, that contain the trigger condition “Whenever you play a development from your hand”, must be played in response to playing a development from your hand.