I like Triangle Strategys way of doing it. At anytime during dialog you can press a button to speed dialog up, have it autoplay so you don't need to keep hitting 'a', and/or you can just skip it all together.

If your dialog is so important that you think it cannot possibly be skipped, put it in a note or journal entry after the dialogue. And let us skip it if we want.

Owl from Ocarina of Time flashbacks:
“Did you not get the fact that maybe you didn’t not get that but not THAT?” (YES / NO)

Yahtzee (Zero Punctuation) had a walk and talk vid that was awesome: https://youtu.be/sZDARCBm_gg?si=lYyji5hrXVkSSwr7

There's an example of a game (in the vid) where they can interrupt the dialogue and have them respond. I think it's Bionic Commando remake and I think that's a nice touch.

My favorite (and quickly becoming the only sort I can tolerate) of npc dialog is where it shows the scene of them speaking with the voice over, but also shows the subtitles and you can skip to the end of the sentence without skipping the whole thing. Now when I play a game with skippable cutscenes that don't do that I have to physically hold myself back from trying to skip it, and if it's unskippable altogether I just get ptsd.