The script is interpreted, not compiled, so it is read line by line. Thus, if you edit whatever is not yet in the buffer, it will affect the execution of the script.
It was not clear from your question, but do you actually want to be able to change your script during execution (by another script, for example)? As you said -- don't do that! If you just want to make manual changes to the script so that you don't forget about them while it's running, just copy the file, edit the copy, and move it back onto the original when it's done running.
Good luck to you on your shell switch-over!
(BTW -- this is not really the "VERY basics")