Take advantage of every moment your protagonist crosses from one symbolic place to another. Every threshold has the potential to alert the reader, audience that the character is transitioning from the known to the unknown = creates excitement, expectancy, and an element of fear of the unknown in both the character and the reader.
The use of crossing a threshold is especially effective when the character moves from the Beginning (1/4) to the Middle (1/2). After the scene that represents the End of the Beginning, the character crosses over into the Middle. This is a big moment and often comes as the character leaves her world over everyday ordinary for the exotic world of the unknown.
Another symbolic moment is when the character leaves the Middle and moves into the End (1/4). You will likely find other thresholds or transition to take advantage of in your story.
This moment of crossing over the threshold deserves a emphasis with the use of pacing and introduction of authentic details and "showing" (not telling) the character's emotional anticipation of the moment of crossing, the actual sensations as it happens, and possibly the reaction when the action is complete and the character understands there is no turning back.
The character will likely be confronted with Threshold Guardians, but for now, go through your manuscript and locate any sections that could constitute a threshold. Rewrite that section to give it the significance it deserves.
For more on Crossing the Threshold:
