How does extra damage work for critical hits?
I'm a bit confused on how extra damage works for critical hits. Let's take the following example:
A thief with sneak attack rolls a crit. I'm under the impression he rolls double his normal attack dice AND double his sneak attack dice. Is this accurate?
Another example, I finished DM-ing my first campaign recently. The Giant Spider enemy monster has an melee attack that causes a constitution saving throw. If the player fails, the enemy rolls an additional 2D8 of damage. In this situation, if there was a critical hit, and the player failed their saving throw, would I also roll double dice on the additional damage? Or is that considered a separate attack?
From the Player's Basic Rules:
When you score a critical hit, you get to roll extra dice for the attack’s damage against the target. Roll all of the attack’s damage dice twice and add them together. Then add any relevant modifiers as normal. To speed up play, you can roll all the damage dice at once.
For example, if you score a critical hit with a dagger, roll 2d4 for the damage, rather than 1d4, and then add your relevant ability modifier. If the attack involves other damage dice, such as from the rogue’s Sneak Attack feature, you roll those dice twice as well. (PHB, p. 196; BR, p. 75)
The example given answers your first question perfectly.
For the spider, the secondary damage is from poison, so is not inherently part of the attack. This can be seen by the fact that the damage doesn't depend on the attack roll, but a separate saving throw.
Critical hits represent hitting a vulnerable area. With poison, it doesn't really matter where you are hit.
I would add that some people double the damage rather than rolling extra dice. The very nice analysis here suggests why one should roll the extra dice.
Ah, my apologies. I did not mean to suggest you should update your answer; I can see where the language would suggest that. I was merely attempting to note that the extra damage from a critical hit should be *rolled*, and not merely doubled. I apologize for any confusion in my word choice.