I start with the big picture, the 'so-what?' of the story, set the world up, then work my way backwards. The world is the first 'character' I come up with, and every flesh and blood being living IN it comes after. Tolkien used this method in his writing, and it's an approach I've always admired. I'm of course, by no stretch of the imagination, as good of a writer as he was, but his method is very effective. 

I have run a LOT of Dungeons and Dragons games over the course of my life, and that involves coming up with personalities for my players to interact with; sometimes on the fly. I try to imagine myself in their shoes. What is their relationship with others like? What do they like? What are they afraid of? Then, I boil these attributes down and play their part for my adventures.

For example, when Mikey, the veteran in 'The Flare', hears 'Green Hair Bitch's' footsteps for the first time, I flavored the sound like distant gunshots. That's why he's the first to notice, and why he reacts like he does. I didn't reveal this part of his past right away because I wanted to demonstrate it through his actions. 

'Tales From the Tank' is meant to explore both worlds. I want the reader to understand the feeling of being shrunken and afraid, while also understanding the mindset of a person who sees those shrunken people as mere playthings. 

Personally, I find that the best giantess stories are a balanced mix of horror and erotica. The situations have to be a mix of terror and arousal, the beautiful and the grotesque. If there isn't any risk, the fantasy becomes dull. If the violence is too gratuitous, there's no heart.