displaying dialog over a rectangle.. not talking about the dialog area for selecting dialog

How would i go about adding a rectangle underneath "spoken" dialog similar to how the dialog area works? since i dont mean adding the dialog area that is used for selecting dialog to choose from.. i mean dialog that is displayed after choosing a dialog piece... and an npc "speaks"

right now i have the dialog show on the bottom of the screen.. which is how id like to keep it, but with a dialog box that is underneath the dialog text..

Create a dialog background in an interface & show / hide it as required with action parts before & after display texts.

You could of course use the text handler functions via Lua script to call & hide it automatically each time a text begins & / or ends, but that method may have the background flicking on & off between each display text.