SOPP264: Types of liturgical pieces for organ

This question was sent by David. And he writes: Dear Vidas and Ausra. It seems to me that there are basically 5 types of music that the organist has to be ready to play in church other than accompanying hymns and choir anthems. They are: Preludes, Postludes, offertories, introits, and interludes. Perhaps interludes aren't so common in most churches now. What are the important aspects to know about each of those in order for the organist to select or improvise an appropriate piece of music for each? Are Preludes usually longer, more meditative? Of course we know that Postludes must be played as loud as possible to prevent idle and rude chatter while the organ is playing (I'm joking, of course). But what makes a piece more suited for a Prelude, and another piece more suited for postlude. Do offertories have special characteristics? When a church uses them, what is appropriate for an introit? Are there any special guidelines that generally can be applied? Obviously every church and denomination is different, and differing themes and seasons will affect this, but I'm looking for general principals for the average service or Mass. Thank you, David