Maybe we'll have something like a pop up asking if we want the reaction to trigger or not. But in this case it's a little bit useless to enable it before the end of your turn...
Really hope this TB is going to be more "interactive" for players and AI during other's turns. That could be a perfect mix between the classic TB games and the RTWP/D&D experience.
The Solasta demo pops up a yes/no dialog asking if you want to use your reaction, which might seem quite intrusive to some, as it stops the game. An alternative would be a transient dialog up for a few seconds "press to activate shield" or "press to acivate attack of opportunity", or something similar.
I think their design is because they didn't want to stop MP every time a reaction was possible, but I think a transient system could work well.
Yes that's really what I imagine... A small window pop up for a few second with the different reactions player's can enable. We can also imagine time slowing down (for... let's say 5 sec) during the ennemie's animations if the player have something to do. That way you don't have the feeling of another cut in the action, which is important in TB.