
Originally Posted by
Dragonfree
As with so much else, this is about properly imagining your characters as people whose views and opinions emerge organically from their experiences and general philosophies on life, instead of as plot devices that say and do whatever they need to say and do in order to make the story you've planned happen. If you have a whole and complete idea of who they are, what they care about and how they perceive the world, you can imagine how that opinion fits into their worldview, why they hold it and how they justify it to themselves, provided the opinion makes any sense at all for them to have.
It's also a good exercise in general to train yourself in everyday life to really listen to people you disagree with, consider the points they make fairly and try to understand where they're coming from. Often you can dissect a disagreement down to a simple fundamental difference in priorities, for instance, which should make it fairly straightforward to simulate someone who holds the opposite opinion, since you can just derive it from the fact they care more about X than Y. Sometimes it comes down to a difference in perceived truths: if two people have been led to believe different statements about the world are true, they can end up with completely different opinions even if they're both thinking absolutely logically. And even if they believe something that is in fact simply and plainly fallacious, it fits somewhere into their cognitive framework, as part of an ideology or vision or wish or justification for something else. You can make great use of all of these kinds of opinions in characterization, and if you've observed closely exactly what lies behind people's disagreements with you, you have easy inspiration for writing characters whose worldviews are strongly opposed to yours.
It should only be inspiration, though - your characters should believe things for reasons that make sense for them, not just for any random reason you've heard people use. That applies to characters who agree with you, too - they may believe some of the same things you do, but that doesn't mean they believe it for the same reasons and would use the same arguments to defend their position.