I don't have my Premiere now but I'll will try to see whether I remember. Open new project in PE and put your clip with talking head on layer 2. Open effects and find Chroma key and drag it onto your clip. This effect should remove all green pixels and leave that part of screen trensparent. If not immediately, choose yourself the color (green) on your clip with a small picker after finding that effect in applied effects-Chroma key and play with the sliders (Similarity Blend etc) until you remove those pixels. Now when there is only head on screen use your background video or still and put it on a leyer below (layer 1). Now you will be able to see new background through erased part of talking head clip.
I think that almost the same effect will be achieved with the effect Videomerge and sometimes the software will reckognize solid color on yout clip and ask whether you want it removed.