DM style questions. Just looking for insights and justifications on different approaches.
Case 1)
I've seen players and DMs alike resolve actions simultaneously when, by RAW, it should all be sequential. For example, a player with Extra Attack will announce, "I'm going to take a couple swings at the Ogre," then roll 2d20. It's expedient, I get that, especially for a big bag of meat like an ogre. Let's say both attacks hit, but you only needed one hit to KO him. Do you let the warrior move and keep the second attack roll to apply against another target? Ask for a reroll? Tell him the second attack was apparently spent hacking the monster into red mist?
At my tables, I enforce sequential resolution so it's not an issue. However, I'm a player at a table where some of the players do simultaneous attacks. The DM rides with it, and while it's not a deal breaker for me, it always bugs me.
Case 2)
DM: "Okay Bob, you said you wanted to sneak ahead and scout. Give me a Dexterity stealth roll."
Bob: Rolls, "15 total."
Jon: "Oh, I wanted to use Guidance before he snuck off"
or
DM: "A guard turns the corner unexpectedly and looks at the two of you sternly. You and he all know you're not supposed to be in this part of the palace."
Bob: "I make up a story about getting lost."
DM: "Roll Charisma deception."
Bob: Rolls, "5 total"
Tom: "Hold on, I'm double-talking the guard to throw him off balance, granting advantage to Bob's roll."
In both cases, we're looking at the rapid mechanical resolution of an action before the other player got a chance to announce he wanted to render aid.
Do you let Jon cast guidance? Do you let Bob gain advantange from Tom's help? If so, does Bob roll an additional d20, or does he roll a new pair of d20s? Do you first look at the result of the current die roll to see if a bonus or advantage would even make a difference? Or perhaps you expect your players to be on their toes for such opportunities, and it's too late once the die is cast and the total declared?