I see the emotions as to some degree driven to how Riley would naturally feel in the moment and the other emotions fighting them as Riley attempting to feel something else. For example, when I’m embarrassed but don’t want to show it, I sometimes instead find myself trying to get angry by blaming others for the situation. I would see this as anger wrestling the controls away from embarrassment (I know that’s not one of the emotions in the movie, just go with it). So in the movie Riley needs to feel sad and grieve the life she had before, so sadness is drawn to the memories of that life, because the life embodied by those memories is gone and that’s sad. But Riley doesn’t want to feel sadness, so her mind scrambles for other emotions. So many of the triggers she uses for her joy are bittersweet now because they’re part of her old life, so instead other, situationally inappropriate, emotions come out. Sometimes, especially when you’re young, you need permission to feel your feelings. Riley considered herself a happy person, so she kept trying to be happy, even when it didn’t come naturally, until she reached a point where she realized that just wasn’t tenable anymore and let herself just be sad for a bit. No distractions, no qualifiers, not until she was ready. As to Riley’s emotions being all over the place, I’d say that is because she’s a child. When we’re young we often don’t have the skill, experience, or mental development to manage our emotions. As we grow up we hopefully learn that control and our emotions more cohesively form our personality. Great reaction and I’d certainly watch reactions to the Inside Out 2 promotional materials

the reason sadness was behaving the way she was is because her emotions are being repressed. Joy wasn't letting Riley feel the sadness she needed to about missing her old life. I think it was trying to talk about what feeling depression is like as she even said something is wrong with me and she couldn't explain it herself. In fact what Joy was doing would be called toxic positivity. As not just with sadness but she wouldn't let any of the other emotions take control. But the point is you can't be happy all the time and you can't be sad either, you need both to function. which is why at the end of the movie when Joy finally let sadness take control Riley felt better and let it all out. Her core memories are from her old life and even though they were happy, sadness wanting to touch them is showing she needs to feel and grieve.

Well that was necessary