Quest designing is fun.  That might just be the dungeon master in me talking, but I recently spent an hour or two on writing a 3-page quest structure for a very simplistic quest and found the process I take very natural and I think is something many people already do that's worth sharing.

Note: The approach to design in this article can be used for any type of game quest but I think really shows it's power in open world games.

Let's setup a running example to use in this article.  First, we'll loosely use the context of a game like The Elder Scrolls: Oblivion as the game is more blatantly quest driven than some others.  There won't be any direct Oblivion references but it helps to imagine this in that setting.  Let's introduce 3 non-player characters (NPC):

John - Treasure hunter
Nolan - Archaeologist
Sarah - Barmaid, Nolan's love interest

The Process

Step 1: Create the happy path.  Happy path is a very simple quest overview of all main quest steps.  So, using our example, let's create a situation that includes all the above characters.

1) Player meets John in a local bar and learns John is searching for a local legend, a large red ruby.  John asks the player to research information for him (quest given).

2) Player meets Nolan, a local archaeologist who, after conversing, is revealed to have found a large ruby on a recent dig.  Being the shy, hopeless romantic nerd he is, Nolan entrusts the player with the ruby and asks for him to deliver it to his love interest Sarah, the local barmaid.

3) The player meets Sarah, gives her the ruby, and she writes a letter for the player to give back to Nolan.

4) Player returns to Nolan, gives him the letter (in which Sarah confesses her reciprocated love), and Nolan awards the player some money.

Step 2: Add details.  Taking each step, go into more details including things like dialogue options, NPC dialogue, and changes in the game world (things like receiving or removing inventory items, updating quests, etc...)

1) Player meets John in a local bar and learns John is searching for a local legend, a large red ruby.  John asks the player to research information for him (quest given).
    - Treasure Hunting
        - "My name is John and I am a travelling treasure hunter.  I've been searching for the local treasure Frederick's Ruby"
    - Frederick's Ruby
        - "Frederick's Ruby is a local legend, a great red ruby the size of my head!  Would you help me find out more about it?"
    - Yes, I'll help
        - "Great!  Maybe start by talking with the locals to see what they know."
            - Quest Update: I met John, a travelling treasure hunter in the Brown Steed Tavern.  He's asked that I help him learn more about Frederick's Ruby, a local legendary gem.  Maybe the local archaeologist knows something...
    - No, I'm too busy
        - "Oh well, let me know when you have more time."

2) Player meets Nolan, a local archaeologist who, after conversing, is revealed to have found a large ruby on a recent dig.  Being the hopeless romantic nerd he is, Nolan entrusts the player with the ruby and asks for him to deliver it to his love interest Sarah, the local barmaid.
    - Frederick's Ruby
        - "Ah the local legend, I did happen to find a large ruby recently but I... uh... have plans for it."
    - Ruby Plans
        - "I am in love with Sarah, the barmaid at the Brown Steed and want to give it to her as a token of my love but I'm too shy.  Say, would you help me give this ruby to her?"
    - Of course I'll help
        - "Thank the Nine, finally a trustworthy soul.  Here's the gem, take it to her at once and tell her I love her.  I hope she feels the same!"
        - Item Received: Frederick's Ruby
        - Quest Update: Nolan the archaeologist gave me Frederick's Ruby to deliver to the barmaid Sarah at the Brown Steed.

And so on...  In order to make this example short enough I'll skip all the details of the last two quest steps but you get the idea.  Items above in italics are conversation topics for the player to select, bold items are game changers, and items in "quotes" are NPC dialogue responses.

Step 3: Design like a player (or, the What If step).  This step is key to creating a 'tree' quest structure where scenarios branch off a standard quest line.  In this step, you need to think outside of your original design and see flaws or conditions.  Typically they show up in the form of linearity and can be brought up by asking questions.  Here are some questions we can ask about the above quest line:

- What if the player returned Frederick's Ruby to John instead of Sarah?
- What if the player gave Sarah the ruby at the tavern while John is present?
- What if the player gave Sarah the ruby and told her it was from the Player instead of Nolan?
- What if the player declines Nolan's offer but returns to John and tells him of Nolan's find?

All these questions can lead to different scenarios and in some cases could spawn more quests.  Obviously we can't answer EVERY 'what if' scenario but you should be able to see how much more realistic our quest is going to become when we do Step 4...

Step 4: Branch off and repeat (a modified) Step 1.  In this step you take one of the what if scenarios you would like to add and take it through this process again.  Obviously Step 1 is slightly modified because you aren't writing the major points of the quest, you're writing the major points of a branched scenario.

In our example instead of agreeing to deliver the ruby to Sarah in quest step 2, assume the player says no.  Instead, they return to John and deliver the news that Nolan has Frederick's Ruby.  John smiles with his crooked teeth showing and rewards the player 50 septims (coins), darts out of the tavern, and before you know it, Nolan is dead and the ruby (and John) are gone.


Conclusion
The core of designing this way is very common, iteration.  Iterating over design to make it better is pretty much a cornerstone for solid design and in this case it really shows how you can iterate over 4 steps to create detailed and rich quest lines.  This method can be used whether it's for a core goal of slaying Glorgnak the world destroyer or simply fetching an item.

A few items to try: write a quest line where there are 3 or more branching areas with differing ending conditions.  Also, try a quest line where there are several branching areas that end up coming out to the same conclusion.

Finally, here is a sample of what the final quest structure could look like (missing some follow-up items based on how the quest ends and iterative conversation items):