Lets say I have a circle with a fill and stroke. How can I separate the stroke from the fill?
1- Select the object.
2- Choose Object > Expand.
3- (Optional) : Ungroup
Be aware that elements might overlap.
If you don't want that, use pathinfinder "Trim" or "merge" option after step 2
There's a few ways to interpret what you're asking which could be clarified if you say what your intent is.
@Vinny gave a good answer to what I think you may have been requesting. I'll add a (possibly) simpler method to do a similar task:
Duplicate your shape. You can just copy and paste it.
In your original shape, set fill to nothing.
In the new shape, set stroke to nothing.
You now have the same shape in its two different forms. If you need something more complex than this, I'd suggest writing questions with more specifics in them.