Casting a spell before tapping lands?

[Phoenix X30]

someone recently told me you can cast a spell then pay its mana cost. I always thought the mana had to be paid up front.
I got told because someone was playing silence and they told me to cast the spell then pay the mana cost because it's on the stack before silence resolves.

Seems weird.

[particle]

someone recently told me you can cast a spell then pay its mana cost. I always thought the mana had to be paid up front.
I got told because someone was playing silence and they told me to cast the spell then pay the mana cost because it's on the stack before silence resolves.

Seems weird.

The wording is off but the actions are correct. You haven't "casted" the spell until you've named targets, paid costs, and put it on the stack. However, you do "announce" the spell first, then choose targets, then pay costs. This is important because cards like {icefall regent} can change the amount of the cost which is why targets must be announced first. You wouldn't pay the cost, then see its targeting regent and then pay 2 more.  You would target the creature, total the cost, then pay the cost.

[Avodroc13]

technically true, you announce the spell and all valid target involved, move it from hand to "stack", pay appropriate costs for it etc.

no matter the order, if it gets countered you still have to pay for it.

So what about {Fireball}? I state I'm casting {Fireball} then I choose said creature. My opponent gets priority to add, say, a {giant growth} to it. Once it adds, I can then pay it's toughness + 3 (to accommodate my opponent's response)  to kill the creature? I'm not understanding this.

[rarehuntertay]

No. You do not get to pay extra. You announced your spell, declared target, paid cost, which is then locked in. You pass priority to opponent.
They cast  {Giant Growth} in response, targeting said creature. He passed priority and you have no response to that.
Stack resolves:
Giant Growth resolves, giving 3 toughness to creature. Priority yadayada again.
Fireball goes off for what it was locked in for.
Creature probably lives with X amount of damage marked on it.

[Avodroc13]

No. You do not get to pay extra. You announced your spell, declared target, paid cost, which is then locked in. You pass priority to opponent.
They cast  {Giant Growth} in response, targeting said creature. He passed priority and you have no response to that.
Stack resolves:
Giant Growth resolves, giving 3 toughness to creature. Priority yadayada again.
Fireball goes off for what it was locked in for.
Creature probably lives with X amount of damage marked on it.

That's what I thought, but reading this thread confused me.