Fall of the Titans

• WWizards of the Coast2016-01-22
  Official Ruling
  If an instant or sorcery spell cast for its surge cost is copied, the copy is also considered to have had its surge cost paid.
• WWizards of the Coast2016-01-22
  Official Ruling
  Fall of the Titan's mana value is based on its mana cost of , even if you're casting it for its surge cost. For example, if you cast Fall of the Titans for its surge cost and choose 4 for X, its mana value will be 9.
• WWizards of the Coast2016-01-22
  Official Ruling
  The other spell that you or a teammate cast can be one that's resolved, one that was countered, or (for instants with surge) one that's still on the stack.
• WWizards of the Coast2016-01-22
  Official Ruling
  Casting a spell for its surge cost doesn't change its mana cost or its mana value.
• WWizards of the Coast2016-01-22
  Official Ruling
  For some cards, surge represents only an alternative cost, a discount that applies if you or a teammate has cast another spell this turn. Other cards, like Reckless Bushwhacker, have additional abilities or effects if you paid the surge cost to cast the spell.

Rulings via Scryfall