Is everything lost? Can you feel the panic rising and the sick feeling in the stomach? What should you do? Call the church and say you are sick today? Or is there some hope that everything can be played just right? You can feel my answer, can't you?
It still can be done.
I'm not going to get into the details why you are unprepared - family emergency, poor planning and goal setting, multiple distractions over the course of the week, your personal illness etc. What I'm most concerned today is to figure out a way for you to still play the service to the best of your capacity.
First of all, the most important thing is not to panic, not to freak out, and stay focused and calm, breath slowly and deeply no matter what. The internal battles in your mind are just as important to win as the external ones on the organ bench. In fact, they are even more important.
In this scenario I recommend you take a deep look at your sight-reading abilities and think about what can you really sight-read on the moment's notice. Can you sight-read two parts without the pedals or can you sight-read just the tune?
If you can't sight-read the hymn tune, then playing the church service is a little early for you so I assume you can at least do that.
If you can sight-read the tune with your right hand and the bass line with your left - it might be all you need for an emergency like this because the outer parts are the most important ones in many tonal compositions - the soprano is often the most melodically developed voice and the bass - the foundation of harmony.
If you can sight-read only the tune, do this - play the tune with both hands doubling the melody in octaves. This is not an ideal solution but if you play it rhythmically and melodically fluently, the result will be better than you will expect. It's better than trying to sight-read all four parts with many hesitations and mistakes in public.
In both versions, the variety can be achieved with registration changes for separate stanzas - from the full organ, to the principal chorus with and without mixtures, adding the 16' stops and even playing with the reed stops only.
What about the prelude and the postlude? One option would be to take the opening and the closing hymns and improvise two-part preludes where the free part can move in quarter notes, then eighth notes, eighth note triplets, and even sixteenth notes. You can double the length of the piece by flipping the parts and putting the tune in the left hand.
Another option - choose a meter, any four pitches and improvise a short - 1-2 minute piece using imitations between the hands in the character suitable for the liturgy of today. It's easier than it sounds - you just have to move one hand while another is stationary. Repeat the same rhythms in another part. Remember, you only have to operate with four pitches (in any octave). The pedals can play slower note values out of these four pitches. White keys are easier to manage but if you add just one or two accidentals - the music will become more colorful.
If you want to create a piece in ABA form, choose another set of four pitches and improvise one more section. This will be your B part. For recapitulation, return to the first set of four pitches.
By doing this, you can easily improvise an interesting piece for 3-5 minutes (ABA form) or even 6-10 minutes (ABABA form). NOTE: Later sections can become shorter to retain interest and diminish predictability.
Mistakes? There are none. Simply repeat a motive with the same mistake two more times and it will become intentional.
The most important thing is not to think about what you cannot do but about what you can do. It turns out, you have more options than you actually need.
If you can win the internal battle first keeping a calm mind, you have a chance to winning an external one later on the bench.
Did David had a chance against Goliath? Of course, provided he didn't use the strategy and the tactics the biblical giant wanted him to use. In fact, with the new research it's clear that David's victory was very likely scenario.
Next: Improving your success rate
No. 6 Der Winter will hin wichen (p. 29) from Buxheimer Orgelbuch (ca. 1450), a German Renaissance collection of organ music.
Awake, My Heart, With Gladness