I think plan is probably the wrong word for me. I usually have loads of ideas, then allow the plot to drag my character where they should be at the time. Whatever idea fits best given the events that have occurred in the story so far. A good example of that would be my Gunpowder character, Achilles. I originally intended to have him go after Aeternum's character, Jeb, since he thought he was in custody of the girl he was trying to save. Instead, I got intrigued by the idea of doing my first proper JP and ended up JPing with SGAFS(Aqua/Estrello) which instead lead me into a whole new arc with my character. Looking back, I'm pretty happy with how things turned out, since Achilles' redemptive arc has made him much more enjoyable to write. So in short, I make plans, but I'm very flexible about them
In general, I mostly just care about the history part and the personality parts of a SU matching up well. If you're going to write out a well thought out backstory, you better have your character actually reflect it as well. When I write SUs myself, I usually type up history before personality. That's easier than the other way around, imo.
But I also can understand RPs like Trumpets who make backstory an optional part of the SU. If you come up with a personality you really enjoy playing as, you can actually develop the backstory over the course of the RP and find ways to stick it in with the plot even. Several times, I've seen my extravagant backstories fall aside simply because they weren't integrated into the RPs plot well enough to be relevant...
i don't really understand how one can develop a character's history during the RP? unless you mean character development, but isn't that an entirely different matter? it sounds kind of backwards to write up a character's history to suit their development in the RP, i think. i usually have at least a good idea of my characters' origins, family and life events before starting to write, if any. that's not to say i make really elaborate backstories either though... it's usually no more than a basic family setting, unless i'm going for something specific (like in Railways), so they usually still have plenty of room to grow.
I suppose it's not really developing a character's history so much as deciding it after the RP has already started. Of course, you have to decide some barebone things like where they're from and stuff like that but... For example by not specifying, or thinking about it explicitly beforehand, I can throw in that a character's parents are dead or alive when the subject comes up IC, depending on what feels better. Or they might have had some kind of traumatic encounter with this or that as a child if I feel like it. They might be secretly filthy rich-- or filthy poor, and the subject may just have never come up. I give them a personality and that's basically all I decide before I go off into the big wide world. I decide things as I the RP moves along, based on what kind of characters are around mine, and what kind of background would most support the kind of person I end up RPing the character as.
I don't always do it this way-- but I've done it once before with interesting results so I've been trying it more frequently nowadays.