I have been slowly working on Grim Tales (a dark fairy D&D-esque game with an implied setting; sort of my contribution to the
thematic games list) behind the scenes, but it was actually Brendan's
latest post that inspired me to finally work out my Reactions.
Characters are basically either proactive on their turn or reactive in others' turn, but being reactive is just that: you forgo being proactive. I still need to work out the numbers, though, but playtesting will surely help with that.