Why can't I always walk on a field on which somebody is standing?
There are some fields on which only one character can stand. These fields can be easily recognised as they are specially marked, e.g. all fields right in front of depot lockers. Further, if you are involved in a pvp fight, you cannot walk on a field on which one of your opponents is standing. Finally, on hardcore pvp game worlds, you can generally not walk on a field on which another character is already standing.