Yes, the premade characters are just to save your time in case you do not want to create 6 characters one by one. There will be no dialogue, all of them are like CHARNAME in BG series. If you want to play a vanilla IWD without any NPC mod, it will be better if you create 6 characters one by one, as you can customize theirs stat, proficiencies, skills etc.
In case you you are interested, I plan to start another play-through for IWD this year with the following NPCs. Installed recently and have a look of some of them, first impression is more interesting than those I used in 2015 play-through:
https://forums.beamdog.com/discussion/79347/the-new-light-collective-thread-for-lavas-and-aionzs-mods/p1
I enjoy Aionz's NPC written for BGEE and SOD (Sirene, Drake, Aura), the writing is very good and lore friendly. The NPCs are interesting.
I have not tried Lava's BG NPC yet, it will be a good chance for me to try out his IWD NPC first.
I finished IWD with this in 2015:
https://www.pocketplane.net/icewind-dale-npcs/
So IWD have 15 NPCs from MOD which may offer you BG2 like experiences, and if you install the crossmod banters pack, they will talk to each other like BG2 companions~