A hologram is a projection of the outside appearance of a person not in the room.  Best treat it that way: render hologram figure separately on transparent background and use it on a billboard figure in the final scene. Set the 'ghostiness' of the hologram via transparency and blur. Input via ambient only. Diffuse, specular etc. all zero/black. A holographic projection is not affected by scene light. 

Make billboard face camera. Use magnets on the billboard geometry to create distortion of the projection. Play with z-depth render image plugged into the IOR channel to create distortion of the background for the spooky effect. In Firefly try z-map and displacement.  

To turn a scene figure into a semi-transparent hologram you would have to take out all internal geometry and all backfacing polygons; quite a challenge and probably not worth the effort.