If you are sitting in a window seat, the arm rest next to the window will be fixed; However, the arm rest away from the window will pivot upwards.
If you are sitting in an aisle seat, the arm rest next to the aisle may not pivot.
If you are sitting in a seat with other seats either side of you, the arm rests between the seats will pivot up. A certain etiquette comes into play now: Is "your" arm rest the one on your left or on your right? The passenger next to you may not want the rest up.
There may also be some arm rests which do not pivot (next to a bulkhead for example) because the folding tray is in there.