Legit interruptions are my bread and butter, my friend, but in every other instance I have seen them used, they can only be performed on your actions by your opponent. As a general rule of thumb, you cannot interrupt your own action. This prevents the fight from devolving into a "he twitched his thumb so I knew he would attack thusly, so my own action from the beginning of my last post changes to counter it", followed by "I noticed he saw my thumb twitch and his stance changed, so I abandon my original attack and initiate this new one", followed by "his thumb may have twitched, but now his foot has angled slightly, so my own counter changes to this maneuver in order to defeat his new attack" scenario. Thirty posts could cover no more than three seconds of observation with no actual attacks being made, because each writer is constantly interrupting their own characters actions.
Barring an actual interruption from your opponent, each of your posts should follow a linear progression starting at the last action performed in your previous post and moving forward. 'Tis the nature of written combat I'm afraid, as otherwise a match can very easily devolve into a stalemate of "he did this, so I changed that".