Organic vs. Outline

There are two general approaches to starting the writing process: outlining and organic writing. Both methods have advantages and flaws. Many writers use some combination of both. Which is best? The answer largely depends on the writer, but this brief overview should help.

Outlining
The writer undertakes extensive world building and planning before drafting begins. The overall story structure; plus major characters, themes, and plot twists, are outlined.

Advantages

• Reduces the likelihood of omitting important characters/scenes/plot points, etc.
• Gives the writer a road map to fall back on if the story goes astray.
• Theoretically speeds up the actual writing process, which can become as simple as filling in the outline.
• Minimizes the risk of wasting time and effort by becoming disenchanted with the story after writing several chapters.

Flaws

• Can delay the start of writing by encouraging endless world building.
• Promotes excessive exposition on background concepts like magic systems, fantasy world history, character origin stories, etc.
• Runs the risk of turning the art of writing into a sterile, paint-by-numbers exercise.

Popular outline writer: Brandon Sanderson

Organic/Discovery Writing
The author gets an idea and just starts writing about it, letting the story develop organically.

Advantages

• Minimal risk of writer's block due to incessant world building.
• Easier to make changes rather than scrapping a whole story.
• Greater freedom to "follow characters" who take the plot in unexpected directions.
• Lowers the risk of scenes/characters/plot twists feeling forced.

Flaws

• No reference to fall back on. "Working without a safety net".
• Easier to forget important concepts/plot points.
• Increased risk of writer's block from lack of direction.
• Greater chance of a meandering, bloated narrative.
• Avoiding flow/pacing problems takes strong self-discipline.
• Higher risk of continuity errors.

Popular discovery writer: Stephen King

For a more in-depth treatment of this subject, Writing Excuses has an excellent discussion on discovery writing and several on outlining.