Something that I became obsessed with in college was instinct and reflexes within playing. While we work to develop our technique, become more "in control" of our bodies to produce sounds and music, we must also work to develop our instinctual technique. When you hear a sudden drop in sound, what is the instinct? Similar questions can be "answered," or discovered through constantly experimenting, and listening to different versions of pieces, or just any type of music in general. Of course, you have to prepare your music, you have to know the notes, the tempo changes, etc., but the mistake comes in when you are with the group or on the stage and you go on autopilot. Maybe the conductor decides to completely ignore the ritardando, and you are the only one doing it. Maybe you play a solo in an audition at a certain tempo but the hall is too wet, so the tempo you chose makes it sound muddy. If you aren't reacting, through your preparation, then things are more likely to go wrong.
It's probably the scariest, most difficult thing to accept about our development, but it is, in my opinion, one of the most important things about performing that we can grasp. It does leave us vulnerable. It is almost counter intuitive to, basically, be willing to ignore your prepared style and method and simply react to what you hear. Maybe your plan works, maybe it doesn't, but you won't know if your closed off from the moment. Be willing to change, or else it might not sound appropriate or clear, etc. Again, prepare as much as you can. You should have control over everything that you can control, but there are always variables that will come up in the moment. The more comfortable you are with reacting within, or without, your prepared method, the better you will feel when change does come. Don't prepare to play, playing should be a reaction, a combination of your prior knowledge, your preparation and the information available to you at that moment. Don't just go on auto pilot, react and respond; that will make the magic happen.