After rigging a charactor, you should group them for later use, by selecting only meshes, not the armature, and press ‘Ctrl+G’, the meshes will have green outline (instead of default orange). Then rename the group to a meaningful name from the object tab.
Start a new scene, you can link the charactor into this scene by ‘File’ -> ‘Link’ -> ‘Group’, make sure that ‘Link’ and ‘Instance Group’ is selected.
You’ll have 2 new objects in the outliner, the mesh and the rig.
If your 3D cursor is not at the origin, your mesh will appear off its originate position, you must reset it back by pressing ‘Alt+G’ to clear its location.
Then restrict the mesh‘s selection by toggle off its arrow in the outliner, so that you don’t accidentally move it out of place as you would only work with its armature in this scene.
You can enter pose mode and pose the rig anyway you like but you’ll notice that you cannot key any bone. You need to make a proxy of this amature to be able to add any key on it.
So, in 3D view while the rig is selected, press space bar and type in command ‘make proxy’ and click again to confirm. Now, you are able to key this rig.
Notice that the name of the rig would change into ‘_proxy’ and the link icon is disappeared. You can rename the rig to whatever you like.