PLANNING A STORY SIM? HOW?

I am not sure what you mean for a story sim. You mean a visual novel? You should check out Ren'py. It's a pretty neat free visual novel maker. I've never used it myself though so I cannot offer much in the way of specifics.

Yeah, you're thinking of a visual novel, or kinetic novel/interactive fiction variants.

From a planning perspective, I've always started with the key endings and then worked backwards. From there, the individual forks can be determined (which choices lead to which endings) and then the rest of the branches worked out. There's always going to be things that aren't caught in the initial planning phase and just have to dealt with when going back and tying up the loose ends. But anyway, once the endings are there, I've found it's best to make one complete linear route through the story before coming back to the interactive parts.

As for character development... hm. It's pretty similar to a normal story, except usually there's a lot more dialogue in visual novels, at least those that take after dating sims. That's usually a good thing though; it makes it easier to show off a personality and cuts out more boring narrative elements

Oh, also, mostly unrelated, I'm a programmer and (learning) writer interested in developing a VN or other interactive fiction for Android/iPhone or elsewhere if there's an audience. So far the only thing I can claim to my genre resume is this and a WIP, but if you're looking for a collaboration, feel free to contact me via PM.