This is a very good question. I have tried a few different approaches to this problem with mixed results. The short answer, as far as I know, with Revit alone you can't make a stand alone walkthrough.
Good news are there are some cheap and free solutions. If anyone is doing something else please post it as I am always looking for better ways to do this.
1) Use "Blender" which is a free 3D and animation program similar to 3DS Max. Getting the drawing in is a bit trick, I save as a dwg, export individual meshes as sat using a lisp, import sat's into blender using a python routine then start the long process of changing the materials. Then through blender you now have 2 options;
a) Create a guided animation and render the project and create an avi, wmv whatever
b) Create a standalone 'game' that can be run on any computer. However I have had varying success with this and it seems to be very processor/RAM hungry. With the built in game engine you can use a combination of physics and animations. This can be very time consuming.
2) Export to a dwg and import into "Sketchup". However they have removed dwg import from the free version but it's worth the money for the pro as I find it runs smother for walkthroughs. From here you also have 2 options;
a) Save the sketchup file and make sure whoever is presenting or using the file has at least the free sketchup and knows how to use the navigation tools.
b) Buy one of the 3D pdf printers available for blender and email them a 3D pdf. With tweaking you should be able to default it to walking mode. There are a lot of demo's available for this try a few before you buy, you may have to scale your drawing to meters for the walking to work.
Depending on what you are going for Blender will give you a better looking product with more control over lighting, textures, reflectivity and even cell shade options, physics and animations inside the walkthrough. You can even make doors open when you are close or click on them. But this is quite advanced and very time consuming as you get into the cooler things Blender can do. Sketchup is much simpler and you can easily go from Revit to 3D pdf walkthrough in a couple of hours. I often export to Sketchup mid-project and walkthrough to make sure I'm not missing anything because I find it has better navigation options then Revit.
