If I attend a Catholic mass, is it rude not to do the "hand movements", and not respond to the priest when the people have to respond?

No, not at all. Non-Catholics typically remain seated rather than kneel and do not respond with the gestures and verbal responses practiced by Catholics. On the other hand, there is nothing wrong with participating in these words and gestures once you know how what to say and do. The words can be found in the written materials in the pew.

The one action not allowed for non-Catholics is receiving Communion, i.e., eating the body and drinking the blood of Christ. However, you can join the Communion line, fold your arms in front of your chest, and receive a blessing from the priest.

If you are not Catholic, but attend a Catholic mass, it is not rude to omit the hand gestures and you don't have to say anything. Also, you can sit when everyone else kneels.

The only rudeness would be sitting when everyone else stands. Just stand when everyone stands, sit when everyone sits, kneel or sit when everyone kneels.

Oh - also, when everyone goes up to receive the host - do NOT follow them. Just sit in your seat. Lots of Catholics won't go also for various reasons, so you won't even stand out. You are only supposed to eat the wafer if you truly believe it has been turned into the body of Jesus Christ.

No. Same with the Orthodox. The unwritten rule for visitors is pretty much to be polite, but you don't have to do anything you don't want to. Stand when they stand (out of respect), sit when they sit (so you're not a lone candle in the congregation.) Don't make a scene. If you don't know the songs, don't blurt out something else. You know, all the stuff you learned about being polite in kindergarten.

You must respond. You don't have to do all the hand movements, except crossing yourself if you are able.

No.

Because you don't know what to do and it's better than making a mess of it.

If you're not Catholic, they won't be offended, for they'll disclude you in everything they do, anyway, like communion.

I have to go every christmas with my family. Most of us aren't catholic. We don't do the magical hand motions or participate in eating and drinking their god. We aren't the only ones in there to do so, so it's not too strange.

If you're a practicing Catholic, I'd call it more than rude. If you're not, no.

roman catholic sign of the cross invites demons because it's not done correctly. orthodoxy is the only true faith. forgive me.

No it's not rude.