Power Word: Who's the GM?
I have just started a round with a player that suffers from ADHS. I know about the problem. He starts to talk into my descriptions, he talks over other players, sometimes he is hard to get into the reins. But as the GM I have established one small flat ruleset at the beginning of the campaign - some of you may know very similar rules under some term like player-GM/player/table contract. In this case I established two rules:
There's only one person that may interrupt any player at any time for any given reason, and that is the GMfor the exception, see below.
The only person at the table with the right to demand silence from somebody is the GM.
Those two are the main rules, they establish an imbalance in rights, but because of a necessity I learned in games: players will always try to act first or talk over each other or chat two things at the table while the game runs. Rule 1 is mainly meant as a means so any of the players may send a glare to the fellow member that breaks the usually unspoken communual contract of gaming: players take turns. Rule 2 is meant as a last resort, as a means for me as the GM to try to run a game. It's actually the first time that I had to state those rules so blatantly, but until now it has worked just fine.
What about squick?!
We have all had situations where the player of the barbarian pulled out notes to describe in every detail how his axe cuts through the enemy and how he splatters 20 gallons (~80 liters) of blood onto the landscape. Or the Rogue starts to grin and describe where he stabs the caught victim to induce them to spill the information with torture. Or the Ranger speaks about the 1001 uses of a leather strap besides hunting. Or the Bard seduces the dragon, the werewolf and whatever finds his fancy...
I think you get the idea: there are simply things that not everyone wants to have in the game, as such stuff might be too touchy or squicky for their liking.
That is why I give any present person the right to throw in a safeword when things become too touchy for them. Well, it is less a safeword but a gesture: the good old T to call for a Timeout.
Addendum: Table Contracts expanded
Other rules that I found in some table-contracts - even if unspoken - are:
Only one Player shall speak at any time at once.
If you have an urgent thing to say, raise the arm to signify so and you shall be heard as soon as posible.
Picking up the speaking stone signifies that you are to be the next to talk, and that the current player shall finish talking quickly.
Keep off topic to a minimum.
No Rules Lawyering.
It might be worth to look at What is a social contract? too.