Designing a level usually begins with a paper map so you can figure out the flow of the level, size of the encounter spaces and locations of where enemies will approach from. The Designer working on this has a pretty clear picture in their head of what the encounter will be like, but only has a piece of paper to show others. One thing I've found over the years is a good number of people have trouble seeing more than what is right in front of them. So, the Designer has to learn to sell this vision to the viewer and ensure that they see the same thing as what is in the Designer's Head.
Some Designers, like myself, prefer to go ahead with a rough 3D mockup of an area since that helps people get a feel for the scale of areas and a better grasp on the flow of the level. Even with this method, people often have trouble looking past the gray walls to see the finished product that you are envisioning. So, it's really important that you learn to sell this experience.
What you need to help others understand most is the experience that the player will have as they play through this level. So, it's up to you to find the best ways to get this across. I tend to be very animated with my demonstrations and usually even get up and move around the room trying to show the view of the player. I can't draw to save my life, so I'll often have an artist present that I've spoken with about the experience before who can help out by providing a few rough sketches that help sell the area. Since mood is often important, you can also show clips of movies that help demonstrate the feel that you're going for.
The most important thing to remember is that it's up to you to sell the experience so everyone understands and gets on the same page.
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