@s.moradkhan when adding a character to a new stage the timeline for that stage needs to be modified. By default, start and end are both zero.
Adding collisions to dynamic characters is something we are working on currently. If the character is standing in the same location, one option is to add a invisible collision shape like a cylinder around the character. To do this, create a cylinder shape/mesh. under the property tab add->physics->collision. and then set the “purpose” property under raw USD settings in the property tab to “guide”