All reactions should be toggle-able through their icon
(so you can deactivate attacks of opportunity for your mage who should rather counterspell).
Most reactions should be triggered automatically if toggled on because they would be too bothersome to activate manually
(like attacks of opportunity, protection fighting style or the shield spell).
The first possible reaction is performed, no choice to not perform it if the toggle was on (to not slow down combat).
Counterspell could be activated through a glowing "interrupt" button that appears during a hostile casting animation
(you may specify details in a second menu that pops up, hitting interrupt is the only thing that needs to be done within a short timeframe)
That seems like a lot of fiddling to save time. I know people usually roll both to hit and damage dice to speed up combat so I understand the mentality of "every second counts" but I'm starting to think at which point do we draw the line...
