Well, this will be interesting :fufu:
I believe nearly 100% of the roleplays I've tried in my life were long-form forum roleplays, so I'm not sure how helpful I will be. But I can say that there are a few things that may actually throw me off about short-form text...
The main thing is, I've noticed that I tend to write some of my posts as if they were some mini stories or acts of their own, trying to give them some closure and only then adding a cliffhanger if it's needed. That may really just become a bad habit for short-form because I'd have to be faster with writing posts and really let the overall setting and plot drive my own writing, instead of adding lots of personal elements and references to my character's own story.
But that's exactly what I like to do, and sometimes even feel obligated to do, in order to make my character feel more defined, interesting, and involved in the actual setting of the roleplay! And here's the interesting point - I feel like even in a short-form roleplay I'd probably be drawn, like a magnet, towards those longer and more detailed posts. And of course, I'd have to try my best to make those posts interesting and fresh each time, thus adding to the nuance, the things I need to add, the length of the posts ^^;
I have to say, though: I don't think that a roleplay, just because it's short-form, will require less character planning. Or rather, that shouldn't feel like an excuse... even in a short-form roleplay, where I know that - presumably - I'll have less time to indulge in self-reflection or anything like that, I'd want to make sure that I know my character well, maybe set a few goals for them to accomplish or work towards, and be consistent with how they react to what happens around them. Those are all things that help a lot when writing the actual posts, no matter what those end up being like!