Some people seem to be perfectly at ease when improvising. This uncertainty doesn't seem to bother them at all. On the contrary, they seek out opportunities to improvise, they even start by playing a written piece and later expand it and finish it by improvising.
So what can you learn from improvisers? Are there any things that can be applied to your performance even if you don't improvise, even if you only play music from the score. Here are some things that might be helpful.
Focus on what you know. When you sit down to improvise and are given a theme for strict improvisation, you might be afraid of not being able to do it in the correct manner. Instead, you relax and play what you know. You might not know how to play in a contrapuntal style but the modal techniques might work for you.
The same can be said about playing from the score. Out of several pieces you are working on right now there surely must be your favorite one. Over time the list of compositions you are comfortable with will become longer and longer, just like the techniques and tricks for improvisation.
Enjoy the ride. So many people suffer from performance anxiety. The primary reason for this is where you put your focus during the performance. If you focus on yourself (inward), then it's really frightening because others are watching you and supposedly judging you. It might even be more true when you sit down to improvise. You might be thinking that your listeners will notice your shortcomings.
Instead, you should focus your attention on the music (outward). Simply know that your listeners came with good intentions. They want to enjoy your performance. So should you. Every moment you spend on the organ bench, you bring great joy to some. This feeling always relaxes the pressure and helps treasure the moment.
Tell stories. Improvisers always tell stories. The story might not be easily understood by the inexperienced listener because it might be a musical story. Just like creating a verbal story, we use words and language to communicate, so do improvisers who use sounds and musical language when they create music.
You could use the same idea when you perform from the score. Figure out what kind of musical story can be created out of this piece. Analyse the tonal plan, thematic development, melodies, rhythms, harmonies, texture, and form and be conscious about at least some of these musical elements when you perform in public. Don't just play the notes. Put some meaning into them.
Make mistakes. This one might be the hardest to accept but it's vital, I think. Improvisers always make mistakes. People just don't notice them. Sometimes when the mistakes are noticeable, improvisers incorporate them into a piece. Improvisers train themselves to use whatever is at hand, including the mistake because it might and it will lead you to some interesting musical adventures.
Are you afraid of making mistakes when playing from the score? You shouldn't be.
The thing is, if you are not making mistakes, you are not doing anything that matters, you are not creating value, you are not reaching your full potential.
And the twist is (because there's always a twist) that when you allow yourself to make a mistake, when it's OK to fail, you suddenly notice that mistakes disappear. Isn't this the case when you play alone so relaxed and nobody is watching you? When failure is an option, so is the success.
Keep these things in mind when you perform in public. I know, it takes completely different mindset to be relaxed and assume improvisational stance but it makes all the difference. Just remember that the vast majority of organ music composed up to 1800's was created with the idea that it might serve as model, as an example for improvisations.
It actually is fun to create musical conversations (whether written down or not) when somebody is watching you. Isn't it the same of how we feel when we act in front of others? Humans need audience.
The voices in the piece become living characters that can interact, mimic, scream, weep, cheat, fight and do the things that people normally do. Then stage fright becomes a distant memory in the back of your mind which will not bother you during performance.