In?

There are times when one line is enough to bring you inside a story or a new chapter of a novel.  Something about the wording and deployment releases that charged sene of reality.  In a real sense, you've thought about and toyed with this quality of being in for most of your life.

Even when the opening line comes from a book written in the nineteenth century, on top of which you've attempted to read your way through the twentieth century and into the present one, you still feel the charge of being in.  

Add to that the fact of certain nineteenth century novels having been written by an author your own favorite writer of all time has made fun of, in is in.  You recognize it, strive to achieve it, spend time trying to describe to yourself so there can be no mistaking the in-ness or out-ness of something you're reading or writing.

Being in causes a different sense of and regard for Reality.  Perhaps you oversimplify when you say being in, whether you mean in a work of another writer, in work you are in the act of capturing, or neither of those, yet in the experience of a particular musical composition or random moment, means you and the experience have become one.

The goal is to reach in under all these circumstances and as many others with relevance to the way you look at the world, its phenomena, its living beings.. Sometimes, you are several paragraphs or pages in before you encounter a single work with the power to knock you out with some kind of question or thought.

Then you see how fragile the in-type reality is, how you must watch with great care.  A bit too much description.  One unnecessary word in an exchange of dialogue.  A metaphor of enough complexity to cause you to stop, spread it all out, then try to determine its purpose.