Wesley's Weekly HOW TO: Ghost Effect
Making a ghost effect is one of the oldest tricks in special effects, but before the use of Optical Printers and green screens, effects makers used a trick known as "Pepper's Ghost" in which a semi-transparent mirror was used to reflect an image of a person standing off-camera.
They would paint the space behind the actor solid black, and shine a spot light on him, and get him at such an angle that the Camera would see the ghost image transposed on the scene, but would frame out the guy standing off frame.
This isn't of course how we are going to do this effect, but we're not going to be far off. They were doing the same thing we are doing now, which is taking one layer and and transposing it it on top of another layer of footage.
For this effect you will need:
1. A Green Screen.
2. An Actor or Puppet to put in front of the green screen.
3. And en editing program that can do the Chroma Key effect.
You start out by filming your actor doing the lines in front of a green screen, THEN you just film at a similar angle the background without your actor in the frame, where the actor is supposed to be standing.
It helps to film the background AS LONG as your actor is going to be talking; if the shot is 3 minutes long, film for 3 minutes. I found that if you film only say 10 seconds of footage and tell it to just repeat on the background, you CAN get glitches and jitters in the background whenever the background layer repeats, so just something to watch out for.
So now that you have your actor in front of your green screen, and the background, it's time to import the footage into whatever editor you're using.
Put the Actor on the top layer, and the background on the second layer.
For more information on Green Screening see Steve Nelson's wonderful Tutorial here.
HOW TO KEY IT OUT IN AFTER EFFECTS:
1. Open your footage. 
2. EFFECT > KEYING > COLOR KEY
3. Mess around with it a bit until it looks right, Adjusting the tolerance and Edge Feather. It doesn't have to look perfect, and setting the Edge feather REALLY high can give a fuzzy halo look around your actor, which might make them look more "Ghostly."
4. Move your background footage in and see how it looked behind your actor.
NOW FOR THE GHOST EFFECT:
Now that you have them cut out on their own transparent layer, in After Effects you're going to want to color correct it, giving it a nice solid color, like green, or blue helps, and maybe turn up the brightness a little, turn down the contrast, or add some blur. 
This is what mine looked like in After Effects, this was just real quick.
What I did, was once I keyed it out, I went to
1. "Effect > Blur & Sharpen > Radial Blur". I like how it makes it look, I turned it down, and just added it very lightly.
2. "Effect > Color Balance > Color Correction" I turned up the blue on this, you can easily color it ANY color you want.
3. "Effect > Stylize > Glow" Just mess with it until you like how it looks. Glow is invaluable because depending on your settings you can adjust this for almost any kind of monster or apparition, very light, very bright for Angels, dark and high contrast for Demons, you can add in digital effects, and mess with the "Glow Dimensions" and really make it look interesting.
THEN you right click on your footage, you go to "TRANSFORM > OPACITY."
I set mine to 80%, I could have easily set it lower or higher depending on how I wanted it to look.
Now all you do is render it out, and edit it however you want.
HOW TO DO IT IN SONY VEGAS:
Import your footage just like you would in any other software. Of course putting your background footage and your key footage on different layers, Key footage on top.
To Key in Vegas:
1. Right Click your Top Footage and click on "VIDEO EVENT FX..." it should bring you to a window that has a lot of things in it with SONY in title. About halfway down the first column you'll see SONY CHROMA KEYER, double click it. 
Click the Dropper Icon, click your green screen in your preview window, and then mess with the threshold and the blur until you're happy.
There are also some Presets at the top for almost any color that works pretty well.
Once again it doesn't have to be perfect, because a slight halo can one again make it look more ghostly. 
Here is what my effect looked like in Vegas.
How I did THIS ONE is:
1. Right click my top layer, and click "Video Event FX...", since it already has an "EFFECT" on it, since you keyed it out, you're going to have to add another to the chain, click the "PLUG-IN CHAIN" Icon at the top of the Video FX window, it will be a little Green Shape, right next to the "Delete Plug-in" and the "Question mark" box.
2. You will be in the Video FX selection again, and go down to SONY GLOW close to the bottom of the first column.
3. Mess with Glow a little bit, this is what mine looked like for my effect.
You will probably want to try something different, but this is a good stepping off point.
4. NEXT I just added "SONY LIGHT RAYS" in the exact same way, I messed with it a bit until I liked the glow. 
After that, I liked how it looked, so I rendered it, and edited it, and I think it looked pretty good.
Hope it works out for you, and if you have any questions, just Add a Comment below and I'll try to answer your question.
-Wes
Freak'in awesome!
agreed, congrats wes.
wow, most people just ignore us Vegas users, thx.
People use Vegas??
:P Kidding! I use it too!! Great work Wes!
Another way you can do it that is more similar to the pepper method is to shoot your ghost or transparent object against a black drop I recommend a dark room with a really focused light above the subject. Just be sure to make sure the background is absolutley not seen (registering on your camera as Black), stay as far away from walls or do it outside with the gain set to 0db and your shutter up so all you see is the light on your subject. Then take it into after effects and change the layer treatment to screen. then place that footage over your background. Because the screen method treats transparancy in values of black to white anything that isnt 100% black will show up. but what makes this method so nice, is how it handles the graduation of light from black to white. it really is a faster method of doing a ghost. The biggest draw back is you really have to shoot the subject in a different location so you cant have alot of interaction between say an actor who is not a ghost, and one that is. but it is another method that can be utilized and save people time messing with green screens. I used this method as a way of having hundreds of arms reaching out of walls as an actress ran down the hall trying to escape. the effect was shot both ways, on a green screen and using the lighting method. I tried both but the modified pepper method was faster and looked better.
nice job, wes
Nice work, very helpful.
Wes is the MAN!
If you have backplate footage that is too short, There are a few ways to avoid the "blip" that comes from looping. Try just taking two frames and looping them. With one frame, the noise/grain is frozen, and it'll look weird. With two frames, you have the noise moving, so it won't be so obvious that it's a still frame. You can also just take a single frame and add a noise filter to it, at the cost of looking noisier than the two frame method. If it must be moving, you may also be able to run your backplate forward, and then in reverse to extend it. If the blip comes from changing lighting conditions, rather than a moving camera or moving objects, you can also just use a slow fade to loop back to the start of the back plate.
thats one sexy man all ghosty-like. you can trick or treat at MY house this year.