Playing short-term at another church requires you to do things their way. You don’t want people shaking their heads because they couldn’t figure out what you were doing. Here are the things I recommend taking care of beforehand:
1. Find out who is in charge. Pastor or music director?
2. Check that the hymn numbers in the bulletin are correct. Point out any wrong numbers to whoever is in charge, and let them deal with alerting the congregation.
3. Synchronize your watch with whoever is in charge.
4. Ask what should happen at the service start time. If the service is at 11:00, exactly what do they want to occur at the stroke of 11? Prelude starts then? Prelude concludes by then? Announcements? Striking the hour? Once you have your answer, honor that time at all costs. Honoring time makes you look very good, especially if the church is used to an organist who is always late.
5. Ask if hymns are announced or if you’re just supposed to jump in when it's time for each.
6. Ask if hymns are conducted. And hope they are not. If they are, follow the conductor carefully. Unless s/he has no idea what s/he is doing.
7. Ask about tricky rhythmic spots, such as the fermata in Lasst uns erfreuen or that infernal rhythmic kick in Hymn to joy.
8. Ask if Amens are used, whether printed or not.
9. Offertory and Doxology issues: a) if the Doxology is the Old 100th tune, ask which rhythm they use; b) ask about an Amen; c) ask if you should extend the Doxology introduction so that the ushers can get back down to the front; d) ask what you should do if the offertory is over but the collection is not.
10. Ask if there are any unprinted sung responses in the service. Ask if there are any spots where you need to provide pitches for the choir. Ask if those pitches should be blocked or spelled out.
11. If you’re playing the anthem, ask how many people are in the choir. This will help with organ registration.
12. Ask if there is any music they’d like played on the other instrument. Sometimes churches are shy about asking an organist to play the piano or vice-versa, so offer them that flexibility up front. It only makes you look good, and it gives them a better service.
13. Ask if there are any non-musical tasks you should perform, such as dimming lights, rolling a tape recorder, moving something, etc.
14. Plan where you will sit during the sermon. Try to stay out of sight so that you are not distracting to others. If you end up being on full display or in the line of sight of a camera, sit still, and don't text or play games on your smartphone. But if you're completely out of sight (and I was in one church for 7 years), then bring your smartphone and your laptop and a good book. If you're playing three services, you'll want the distraction. I wrote my dissertation during sermons for months. And I composed a lovely piano four-hand arrangement of [title withheld, because I never got permission to do it] during FOUR Easter sermons one year.