At one point in my life, I was engaged to a protestant guy, and as we began to plan the wedding, one of the underriding (like that?) questions was, "um, how do we explain the protocol of a Catholic Mass to half the guests that aren't Catholic?" Given that I, too, have many non-Catholic friends (in fact they override the Catholic ones), we got this question a lot. And Catholics, of course, feel free to read and comment. And I am basing this on my experience and understand of how the Church operated. Alas, there are parishes that do not follow the below guidelines. Sic their bishop on them!!
So, for posterity, I am recording what to DO and NOT to do if you are not Catholic and yet find yourself in a Catholic Church (note: these are guidelines for a general Mass. There's no way I'm going into all the possible variations for feasts and solemnities and Holy Week and Christmas and what have you. But these'll get you through). I've attended Lutheran services, and they are fairly similar to the Catholic Mass, with a few changes. So here we go:
1. When you enter Mass, you'll notice people dipping their right hand in the holy water fonts by the doors and making the sign of the cross. This is to remind us of our baptism and note that we are entering a holy place. Do it or don't do it--up to you.
2. When a Catholic enters the pew, he will not just slouch in. He will genuflect towards the tabernacle, make the sign of the cross, and then take his seat, acknowledging the True Presence of Christ in the consecreated hosts that are stored in the tabernacle. Again, up to you.
3. Most Catholics will then kneel and pray. Mass is preceded by five minutes of "sacred silence", so this is not the time to ask your neighbor what he thought of the latest Jennifer Aniston flick. Look through the hymnals and books available; one of them will probably have an Order of the Mass, with the congregation's responses in bold, so you can follow along.
4. Mass begins with an opening hymn and procession. Stand up. NOTE: Catholics do not sing in four-part harmony! The quickest way to reveal Protestant roots is to begin singing the Alto/Tenor/ Bass/ your own harmony line to "The Church's One Foundation" or whatever the opening hymn is. This is particularly true if the opening hymn is "A Mighty Fortress". Only Catholic choirs sing four-part--the congregation doesn't. Don't do it.
And Catholics and Protestants do not have the exact same hymnal, so don't expect to know every hymn.
5. Introductory rites: The priest opens the Mass with "The Lord be with you". Then we have the penintential rite ("Lord have Mercy"/ "Christ have Mercy"/ "Lord have Mercy"). Unless it's Lent or Advent, the Gloria ("Glory to God in the highest/ And peace to His people on Earth"), which is sung, follows. This is similar to the "Hymn of Praise" some Protestants sing, although the words are totally different.
6. Then we have the opening prayer. Sit.
7. The Liturgy of the Word: First reading, from the O.T. (or Acts/ Revelation, depending on the feast/ season) Notice "reading", not "lesson". The response after the reading is "Thanks be to God"
8. Responsorial Psalm: Sung by the cantor or small group (if small group, could be in four-part harmony). The cantor will sing the refrain and the congregation repeats it after each verse. It's usually a psalm, but could be another scripture verse.
9. Second reading; an epistle.
10. Gospel Acclamation (stand): Unless it's Lent, some form of "Alleluia". Same protocol as Pslam. If it's Lent, then it's "Praise to you, Lord Jesus Christ / King of endless Glory!" (no Alleluia during Lent)
11. Gospel. The priest/deacon will say, "The Lord be with you". Response: "And also with you". The priest will then announce the reading, which the congregation will reply to with "Glory to you, Lord" while making the sign of the cross on their foreheads, lips, and heart, to remind them to keep Christ always present in these areas. The Gospel is then proclaimed. At the end, the congregation replies, "Praise to you, Lord Jesus Christ." Sit.
12. The Homily, NOT the sermon. Not the most important part of Mass, thus is short. A reflection on the readings of the day and how they apply to our daily lives.
13. Stand for the Creed-usually the Nicene, but occasionally the Apostles'.
14. Intercessions--when the reader/ deacon reads the prayer requests of the faithful. Response is "Lord, hear our prayer."
15. Liturgy of the Eucharist begins--this happens EVERY SUNDAY. The gifts are brought up to the altar, the altar is prepared for Communion, and a hymn is sung. Offetory is also taken at this time. we're sitting at this point.
16. Stand for the preface to the Eucharistic Prayer. Note that this can be chanted. If everyone around you starts chanting (it seems that we Catholics are born with an extra chip in our head that allows us to do monochant without much of a problem), just join in if you're comfortable, and stay silent if you're not.
17. Followed by the "Holy", which is sung.
18. Now everybody kneels (!) for the Eucharistic Prayer. Eucahristic Prayer I is the longest and usually reserved for special feasts. Usually you'll get 2 or 3, which are shorter. This can also be chanted.
19. After the Consecration, the "Memorial Acclaimation" (Christ has died / Christ is risen / Christ will come again) is sung.
20. The Eucharistic Prayer finishes, leading to the Doxology (Though Him / With Him /In Him/ In the Unity of the Holy Spirit / All Glory and Honor is yours, Almight Father / Forever and Ever) followed by the Great Amen.
21. Stand. The "Our Father" is prayed. NOTE: The Catholic "Our Father" does NOT segue right into "for the kingdom". This is the surest sign that there's a Protestant in our midst. :) When I was a cantor in college, we always (well, almost always) had Protestant organists (it was a Protestant school, after all) and the first time they would play for us they would give themselves away by going right into it, stopping abruptly when they realized they were the only ones going on. STOP. We say it after the priest gives a brief prayer.
22. The Sign of Peace. Everyone hugs or shakes hands or kisses (depending on the level of affinity) with the people around them.
23. The "Lamb of God".
24. Kneel again. Time for Communion. The priest distributes Communion to the Eucharistic Ministers and the distribution begins. Hymns are sung throughout, or a piece by the choir. Some churches only do vocal music after Communion is finished as a "song of praise". Whatever.
This is the big sticking point: Catholic doctrine says that you shouldn't receive communion if you're not Catholic. The reason is simple: Catholics see the Eucharist as a sign of their oneness in Faith. When you receive Eucharist, the exchange is thus: the distributor says "The Body/ Blood of Christ" and you answer "Amen". The "Amen" implies that you believe what was just told to you. If you don't, then you're essentially committing blasphemy by taking it. That's not cool. If you're not united with the Catholic Church and don't believe what we believe, then why would you want to receive Eucharist? But some of you will do it anyway, and I will tell you that the Church floor won't open and God won't smite you with thunder (at least I've never seen it) but it's not generally accepted practice. It is quite acceptable to remain in your seat in silent prayer while everyone else receives. You won't be the only one sitting there, trust me. All the little kids'll be with you. :)
25. After Communion, people kneel until the priest/ deacon is seated and the altar servers begin clearing the altar. Sit.
26. Stand. After Communion prayer is prayed. Annoucements (if any) are read.
27. Final blessing.
28. Closing hymn--you stay until the hymn is over!! I don't care if the choir is launching into all 11 verses of "O Sons and Daughters"--you stay!!! It's polite! And it's not the Indy 500 out in the parking lot, anyway!
That, my friends, is the Order and Etiquette of the Catholic Mass. A few other points:
--no gum chewing, please.
--wear nice clothes, i.e. not ripped jeans or a trampy t-shirt or something with inappropriate messages (i.e., a shirt with a Darwin fish wouldn't fly). Remember it's God's house you're visiting.
--Refrain from any and all snide comments about the Faith during Mass!
--Catholics do not bring Bibles to Church--the priest doesn't announce a particular verse that he's preaching on and have everyone open up. You may see some Catholics with St. Joseph's Missals, which give the Order of the Mass and the readings for the particular day. That's not the same thing.
--The Homily will most likely be under 10 minutes. Shocking, I know.
--Catholic Churches don't pass out bulletins with the reading in them. These readings can be found in one of the books in the pews, normally.
Hope this was helpful--Catholics, let me know if I've left anything out.