You can create a background or panel for the Boxes, a portrait for the characters, and a dialog box all on a layer by themselves that doesn't scroll. Make them invisible at startup and set them to visible whenever your player talks to an NPC.
I've uploaded a couple caps as examples, the first one is mostly about adjusting your dialog boxes to fit the size of the dialog, but it also includes a primitive example of a scripting system. (as in movie scripts)
http://www.box.net/shared/flbrtsc86h - Pre-99.42 version.
This second version is an attempt at a better dialog system, it still adjust the text boxes, but you also have three NPC's to talk to in it and theres portraits that swap based on which one it is.
http://www.box.net/shared/8zb2lnnsca - Requires 99.42
Its not a tutorial, but is pretty well commented.