I think I figured out Interception. During your turn, you book and army for interception and, if an enemy unit moves within range, you are prompted for interception. If this is wrong, please advise.
However, I can't figure out reaction. Do you also need to do this during your turn? Or do you wait until the enemy turn?
If anyone could provide a detailed description of the mechanics (mainly of reaction, but also interception to make sure I have it), I'd appreciate it.