when not to use preposition?

English is not my first language,but certain phrases such as ‘stand up’, ‘sit down’ automatically make me to associate preposition ‘up’ an ‘down’ with action ‘sit’ and stand 'respectively.

But in a country of non-native English speakers I often hear sentences which make me wonder which is correct usage?

  1. Do you want to sit? Or Do you want to sit down
    No I get down at the next stop.
  2. Then you can keep standing. Or Then you can keep standing
  1. Both possible, but in the implied context (on a coach, bus, train, etc.) ‘sit down’ would be more usual.
    I get off at the next stop.
    Then you can remain/stay standing. or Then you can remain/stay standing up.