What I find to be an issue with NPCs (and other things) is the number of windows you have to open and close, and especially the description windows.
A first improvement would be to use the same window for all PCs/NPCs descriptions, one replacing the previous one. Instead of a large obstrusive window in the middle of the screen, a smaller one in a corner could do well. Then, possible actions would be accessed from this window, through small icons on an edge for example. It shouldn't remove any possibility since you can only target 1 creature at a time anyway.
The advantage is that when you go from one character to another, you have nothing to close before doing so. Also, You get the description right away with 1 click, along with the rest of the menu we get now.
Expanding from that, an action on the NPC wouldn't add another window, but replace the description/menu one. For example, a click on Trade pops up the trade window and hides the desc/menu. From there, a Done button closes the trade window and shows the desc/menu back, while the Close Window icon closes the trade window, doesn't show the menu back, and maybe even untargets the NPC.
For object descriptions, since there aren't any action available from there (no button need to be pressed), the window would disappear automatically when the mouse gets out of it, that is, if the current pop up under the mouse is still used.