Often a character will have multiple motivations, but they vary in size and urgency. It is the thing the character cannot carry on without having or doing that carry the story. Those secondary needs/wants will often drive the subplot.
The Antagonist or Villain
So what is preventing our hero from reaching his goal? As with the main character's motivations, the obstacles are likely manifold. These road blocks keep the story moving and raise the stakes for the main character. But just as you determined which of the many your character's primary motivation is, determine a hierarchy for the obstacles preventing the character from getting where he or she is going, from inconvenient all the way up to life-threatening.
These obstacles are not always human, or even living beings. While there will usually be contrarian characters in any story, the biggest issue for the main character could be something non-sentient of either internal (disease, phobia, poverty) or external (geography, a political regime, time) provenance.
We considered our own works with respect to obstacles and antagonists.
In Karl's nonfiction work-in-progress, he won't know yet he scope of the challenges until he starts hiking the trails he's researched and will write about. He and his companions could face treacherous weather, health-, geographical-, or animal-related obstacles, or perhaps they may even encounter some interfering humans, as well. (We hope not.)
Sometimes the biggest thing standing in the character's way isn't a traditionally bad thing. For example, in Jerome's work-in-progress, in the process of seeking revenge, the main character falls in love, and he must choose between this love or his original mission.
We talked about how Jim's hero is himself a bit of a villain, yet he is the villain we root for--the best of the worst. Anf this main character is confronting the gamut of obstacles: internal (opium addiction, inhabiting a new body), as well as external human adversaries and the incompetence of his colleagues.
For my part, the baddie in my next work on nonfiction is a gene mutation, so... that gives you a nice wide range of villains to consider. (Lions and romance and genes, oh my!)
Think about stories in which a character's wildest dream comes true (insane wealth, power) and how that lottery-winning event is ultimately his/her undoing. What other unconventional iterations of an antagonist can you think of?
Obstacles within, obstacles without
Instead of workshopping someone's writing this week, we did a check-in on our works-in-progress. In talking about the current status of our projects, we realized that just as there are narrative obstacles for our main characters, so are there narrative obstacles for us as the writers.
For my part, facility with research and medical writing will be a challenge, as well as getting "audience participation" since I'm compiling an oral and written history from members of one side of my family.
For Karl, the white-washing of the geographical and historical records he is researching both motivates his narrative (he's writing an indigenous history of the trails as a counterpoint to the Western explorers' accounts) and complicates his efforts to find information from native sources.
We had a very interesting discussion regarding Jerome's impulse to go back and fix what he has already written before going further. For him, this helps in not making the same writing mistakes again and again. The idea is to correct and carry on better than before.