During services, sometimes we sit and sometimes we stand, depending on the prayer. For some prayers we are required to stand: Amidah (as its name implies), Shofar, Bar'chu, Alenu, etc. But are we ever required to sit? Can someone stand for the duration of the service, or would he be breaking some laws?
All I recall is a commentator saying that we sit during Ashrei because it says "Ashrei yoshvei betecha -- Happy are those who dwell in your house". When you are in your house, you relax, sit down and kick up your shoes. But he did not say you HAVE to sit during Ashrei.