When you have a Voice Over situation you are simply going to separate this dialogue section from the rest and indicate that it is Voice Over. You do this just by adding a VO to the dialogue piece so that the reader understands this immediately. Example:
Thomas (VO)
He still didn’t know that he was being watched from behind the car.
This piece of dialogue just needs to be separated from any back and forth or spoken word dialogue that is happening so people do not get confused. This should not be hard when the Voice Over is accompanying montages, second unit, or non-voice scenes, but when the internal monologue of a speaking character comes into play you may have a tough time.
Make sure you put at least one to two lines in between the Voice Over and any screen direction or dialogue that is spoken out loud. If the character is in the middle of the scene and goes from Voice Over to speaking out loud, you need to indicate this clearly. The key point in all screenwriting is that you just need to be clear to the reader what you are communicating, so once you write in your Voice Over scenes make sure another person reads it and can indicate exactly what is Voice Over and what is not.