we have to stand when the priest is censing (often continues into the first epistle reading - we always have 2 of these) and after that we can sit, unless the gospel is being read or the priest or deacon has said stand, or we are singing (e.g. the Holy God, Holy Mighty, Holy Immortal).
i try not to stand if everyone else is sitting (trying not to be hyperdox!)
also i sit more than everyone else if i have some injury or illness.
about the kneeling on sunday thing, some of the egyptian/sudanese/british etc. copts do it, and i assume it's because people immigrated from countries where their main liturgy was friday because sunday was a normal working day.
one other thing about copts sitting down - many of them are actually in semi prostration position, but the abundance of chairs/pews prevents proper prostration.
in one church i know well, there is a regular english liturgy in the adjoining church hall, and after the sermon (which is after the gospel), all the chairs are pulled away
for the rest of the liturgy. then i see several people prostrating (yes, on a sunday...) when they otherwise would have been sitting in the chairs with their heads bowed.
ethiopians/eritreans in uk are most likely to be seen on their knees (prostration) than in any other posture as far as i can tell!
i don't know why that is, but i also break the 'rule' and kneel (prostrate) on entering an eritrean church as they seem to assume people who don't do this are not orthodox! you have to do it 3 times, and of course you leave your shoes at the church entrance.
i assume ethiopians living in sweden in the winter either wear 3 pairs of socks or have very good heating!
does this frequent prostration in ethiopian/eritrean churches happen in america/africa?