The simplest way to understand this is to realize that you are basically living "inside" the running exe that you're writing.
Contrast this with e.g. C++ where you write a bunch of files, fight with the compiler until they turn into some black box exe, then run said exe and try to guess what it's doing based on the blueprints of your source file. To actually see what's happening inside the exe you would have to run some kind of debugger, but you can't take action based on what you see. You're looking through a glass.
Not so with Lisp/Smalltalk. If you see something wrong you can just fix it on the spot and continue running.
Ah. Given what I already know, it sounds a lot like opening up a Python REPL, importing all the code you need, and then starting the main loop of the program.
> You're looking through a glass.
Heh, through debugger-colored glasses. :)
I see what you mean though. With C++, if you find your problem, you stop the whole show, go back to your source, take a stab at a fix, recompile, and then run once again in the debugger. With Lisp, you're just there the whole time.