Sir Icehawk
Lots and lots of diagrams. I would not aim for a Mass Effect or whatnot as my first attempt at this. I'd go for something more straight forward, which Skyrim really is. Despite the open world, the stories progress fairly on straight lines. It's just you don't have to follow that line. You can veer off into other stories and all.
Hmm, okay I know this is really nitpicking a bit, but Mass Effect's script shouldn't be
too complex. Yes, your character can say different things, but it's not really a dialogue tree in the sense in the sense that 99.9% of the situations are all really linear and have no real consequences, or some sort of consequence which is more or less completely cosmetic. All you would really need to do is script everything
once, then go in and make options for you to be slightly more rude, or a small branch where you can investigate things.
If anything, Skyrim would probably be the harder thing to write, because it being an open world means things aren't necessarily going to happen in the order you think they would. Individual quest-lines could be stored in separate word processing files, but if a certain NPC is involved in more than one quest, then things could get confusing. If, say a character named Khorer Bloodbath initiates a quest where you have to go somewhere and murder someone, only to have him double cross you at the last moment, then you will also have to remember to go back and add in a conditional branch to the dialogue in the quest where Guiladan the Fop wants you to steal Khorer's magic tea kettle.
Bethesda
sort of got around that by just making most of the NPCs exclusive to their respective quest lines, but you just can't always do that, especially when it comes to the civil war, and getting quests from the Jarls. It also kind of means that each character gets less overall development if their role in the story begins and ends at "get this random thing for me," but I guess in a gigantic world like that, you just can't flesh out everyone.
Mabs Kiss
A Rpg ( Think Skryrim type of game )
Well, if you're trying to make a Western RPG, the first thing you need to do when you're writing dialogue for an important NPC is have some sort of system that checks how they greet you. For instance, if you've never met Guiladan the Fop, he would need to say something along the lines of:
"Woo unto me, Khorer's tea is far superior to mine. If only,
if only I had his magic tea kettle."
Then if the player accepted his offer, he would need to say something along the lines of:
"How fareth thee, friend? Have you obtained that tea kettle for me yet?"
And you'd also probably need to check if the player has initially refused to work with him, and then whether or not the quest is finished. Or if you wanted to make things more complex, and give the player more options, you'd also need to check if the player ransomed the tea kettle back to Khorer, and write appropriate dialogue for that, and pretty much any other instance where he would need to greet you separately.