I tend to upvote if someone brings something unique to the discussion. If you’re just giving a quick generic response to OP or parroting the same opinion a dozen other people in the thread have already posted, then I’m gonna read your comment and keep going. But if you provide thoughtful content that promotes discussion, provides a unique perspective, or at least had a lot of thought put into it, I’m likely to upvote it. Same goes for the post itself.
I generally avoid downvoting, even if I disagree with the comment. But if someone is factually wrong, misleading, posting in bad faith, or just being a troll, I’ll downvote their comments in a thread and upvote anyone who attempts to be reasonable with them. If two people are just bickering back and forth, they both get downvotes. And as always, any comments that distract from the discussion or are off-topic will get a downvote.