This rule says that you can start casting it since it has that text:
601.3c. If an effect allows a player to cast a spell as though it had flash only if an alternative or additional cost is paid, that player may begin to cast that spell as though it had flash.
Since the {2} mana is an additional cost, you can pay that on top of Narset's "without paying its mana cost". As such, you are still allowed to cast it as though it has Flash if you pay the additional {2} mana though you can still only do so during your turn in thise case.
Since the {2} mana is an additional cost, you can pay that on top of Narset's "without paying its mana cost". As such, you are still allowed to cast it as though it has Flash if you pay the additional {2} mana though you can still only do so during your turn in thise case.
Is the 2 mana an additional cost? From the way it's phrased, it is just an increase to the original casting cost.
Is the 2 mana an additional cost? From the way it's phrased, it is just an increase to the original casting cost.
For the purposes of determining a spell's total cost, there is no difference between additional costs and cost increases (C.R. 601.2f). Neither is there such a difference for a spell that's cast without paying its mana cost (C.R. 117.9, 117.9d). Thus, for example, there would be no difference to how Rout works in either case if Rout said "...if you pay an additional 2 to cast it" rather than "...if you pay 2 more to cast it".
117.8. Some spells and abilities have additional costs. An additional cost is a cost listed in a spell’s
rules text, or applied to a spell or ability from another effect, that its controller must pay at the same
time they pay the spell’s mana cost or the ability’s activation cost. Note that some additional costs
are listed in keywords; see rule 702.
117.8b Some additional costs are optional.
Private Mod Note
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Why bother with mere rulings when so many answers can be found in the Rules?
For the purposes of determining a spell's total cost, there is no difference between additional costs and cost increases (C.R. 601.2f). Neither is there such a difference for a spell that's cast without paying its mana cost (C.R. 117.9, 117.9d). Thus, for example, there would be no difference to how Rout works in either case if Rout said "...if you pay an additional 2 to cast it" rather than "...if you pay 2 more to cast it".
Interesting; so if, for instance, someone played Thalia, Guardian of Thraben and I cheated out a spell with As Foretold, I would still have to pay the additional 1? It is an additional cost by this metric and it's mandatory.
Yes, Thalia is an additional cost. Or, if you choose to use As Foretold (or maybe Rooftop Storm) to cast your Commander for "free" in Commander you would still have to pay the commander tax.
Narset, As Foretold, and Rooftop Storm (along with a ton of other cards) say "without paying its *mana cost*" which means it takes care of the mana cost and nothing else. Any additional costs are your responsibility to pay.
Narset, Enlightened Master
If I trigger Narset and reveal Rout, can I pay 2 and cast it as an instant without paying the 5 cmc?
Since the {2} mana is an additional cost, you can pay that on top of Narset's "without paying its mana cost". As such, you are still allowed to cast it as though it has Flash if you pay the additional {2} mana though you can still only do so during your turn in thise case.
Is the 2 mana an additional cost? From the way it's phrased, it is just an increase to the original casting cost.
It seems to fit the definition.
Interesting; so if, for instance, someone played Thalia, Guardian of Thraben and I cheated out a spell with As Foretold, I would still have to pay the additional 1? It is an additional cost by this metric and it's mandatory.
Narset, As Foretold, and Rooftop Storm (along with a ton of other cards) say "without paying its *mana cost*" which means it takes care of the mana cost and nothing else. Any additional costs are your responsibility to pay.