Morph / sacrifice / split second

[Rass]

Oops I was too busy laughing to comprehend what was being said

[Tetsomori]

Question:  From the example, can my opponent {Shock} my {Ball lightning} to stop the spell?
Answer: No. By the time they can cast instants or fast effects, the Ball has already been sac'ed. (stop giggling) it's sitting happily in the graveyard by then. 

Lol you like sacing your balls at you opponents face eh bj?

Question 2: When I announce {Fling}, can my opponent {Shock} my Birds to keep me from paying the cost?
Answer 2: No.  From the moment I announce my spell until the time I'm done paying my costs, my opponent has to sit and wait. They cannot just "jump in" with a fast effect.