Question: For a fantasy photo project I would like to create a ghostly atmosphere. What I have in mind is a scene with 2 characters, one of them is "real", so will be completely visible, and the other is a ghost or phantom, and would be more of a shadowy figure on the picture, coming from one side behind the other character and leaving on the other side.

Shall I try a long exposure time, with one character staying still and the other quickly passing through? I'm just afraid it will all be blurry, since my main character will be wearing light colors while the ghostly figure will be wearing dark clothes (possibly a long black cloak or something similar).

I'm working with a Sony A33, a basic 18-55mm and a 50mm 1.8.

Thanks for your help!

Answer: 
Exactly as you say, a long exposure will do the trick... You should use a filter if you are shooting at daytime, otherwise you will perhaps need to compensate the exposure in postprocessing... Your subject (the real one) needs to be very steady so it is sharp while the ghost who is moving will be blurred... Good luck! P.S.: tripod is a must!
I'd suggest combining a long exposure with flash/strobe if you have access to it. This will give you a sharp exposure of the non-ghostly subject. You'll still need to get them to sit still but the result will be much sharper than just using ambient light.

Alternatively if your camera supports it you might be able to double expose a frame. A long exposure for the ghost (using a ND filter if needed) and then a short second exposure with the subject in the frame. If you set up the camera on a tripod (or some sort of support - a table, beanbag, etc.) you should be able to get good results using a double exposure.