Can an action be mirrored or reused? In a scene I linked two groups (object and rig) and appended an action, now I'm experimenting what would be the best workflow to animate more characters in a scene. 

I would like to have a set of reusable actions (preferably shared among the characters) which can be combined in the NLA-editor for different clips.

In this scene the two characters are supposed to stand opposite, the character on right was moved from the position of the yellow empty and changed the orientation because both use the same action. 

Is there a way to mirror an action or add offsets for location and rotation, so that the characters will keep their orientation?
If your action contain some keyframes on the "root" bone's channel, all items you assign this action will be mandatorily aligned.

You should remove the root bone's keyframes from the action, and then animate the roots separately. If needed, you can create separate actions, one per character, and mix them with the first action.