Key what???

In the observations step, one of the last things to look for is the key verse.
The key verse is a verse that summarizes the main idea of the book (it can be more then one verse), it tells why the book was written. This can be done at anytime in the observation method, yet usually it's one of the last things I will look for. By the time you've color coded all your observations, you'll have a good idea what the main point of the book is.

Just a friendly hint, there is one figure of speech in the book of Philemon that is (not the key verse) a very important key to unlocking the reason why the book was written.