There are several ways to trigger dialogue, if your NPC is not global all you have to do is assign your dialogue in the side bar.

However, when you make the NPC global you'll find that you cannot use the previous method.

Here are some ways to make it work:

1. A global script.
Make a new Goal in the Story Editor, name it "Start"
In the KB section, copy paste this then build:

IF
DialogStartRequested( _Char, _Player)
THEN
StartDefaultDialog( _Char, _Player);

You can now assign Dialogue just as before.
=====================================================
If you want more advanced dialogue, this is how you execute it:

Two/Three Speaker Dialogue

The script for a three Speaker Dialogue would be:

IF
DialogStartRequested(CHARACTER_YourNPC1, _Player)
THEN
DialogStartThreeSpeakerDialog("YourDialog",CHARACTER_YourNPC1,CHARACTER_YourNPC2, _Player);

Now, for this all you need is global turned on and the correct character names.

In the dialogue window, there are some important things to remember:
(Assuming you know how to make a basic dialog)

(If it's hard to read, right click and open in new tab)

I hope this helps, if you have any questions, i'm happy to answer.

How do you make the exclamation point appear above a characters head to indicate there is an opportunity for dialog?

Found it. It looks like you loop this effect with:

PlayEffectAtCharacter("FX_GP_ExclamationMark_A",CHARACTER);

I followed your guide, but I can't end dialogues with global characters. If I try the same with non-globals, I can end dialogues just fine. Any suggestions?