Magic: The Gathering Thread XIV: Instant Annoying, Just Add Hexproof

[Bucky]

And technically, few people follow the actual rules for casting spells:
1) Declare you are casting a spell and put it on the stack.
2) Choose all modes for that spell.
3) Choose all targets for that spell.
4) Pay all costs for that spell.

...except that you can float mana for paying the costs before declaring the spell.

And at which point does one get to technically get to do something in response?

For most types of "something", only after step 4. There is one exception; a player can concede during any step. This is the basis for my favorite stump-the-judge question.