Hello isabeau,
I can sympathsize with your feelings of frustration from writers' block and understand why you 'cheated' (I don't mean that to sound negative). It can be just as instructive for you to go through the process of rewriting your story, even if it wasn't written specifically for the assignment.
There are a lot of good ideas throughout your story and it's always refreshing to read a story about tickle torture. There's not enough quality tickling stories out there, if you ask me.
I'm not going to make any specific criticisms at this stage. The major flaw I can see is the way you've written the dialog between your characters. When you have two (or more) characters interacting through dialog, you should always provide separate paragraphs for each so it's clear exactly who is speaking at any given time. Don't be afraid of single line paragraphs.
Example:
"Where am i? Who are you? And what do you want with me?" Crystal asked.
The man, whose piercing eyes had never left Crystal's face, rose slowly at that. "Who i am is of no importance. Where you are is at my cabin, very remote and far from any human. Now as to why you are here, that i plan on finding out soon for myself" the man replied in a thinly veiled disguised voice.
"What do you mean?
Finding out what?" Crystal cried. "Let me go
! How dare you
!" She then began struggling in earnest, etc. (Punctuation corrected in red)
A re-write with this in mind would be a good place to start. It's also a good thing to do to distract your thoughts from the block you're experiencing. In other words, revising an old piece is a way to write your way out of writers' block.
Good luck and have fun
anonymouse