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!
It depends on whether you want the ghost to appear as if it's moving or is still. If you want it to look like it's moving then use long exposure & let the ghost move and use a flash on the normal person so they would get propper exposure. If you want the ghost to appear as if it's just standing there then you can either have soft edges & ghostly transparency by using long exposure & a flash on the human & have the ghost try to stand still during the exposure but not too still or if you want to have a ghost with sharp edges yet ghost transparency you can use two bursts of flash, the first one you'd fire on the ghost & remove the ghost from the scene & then fire the second flash on the human when the exposure is about to end. You can experiment with the last idea depending on the area of the set, for example you might need to fire the flash for the ghost when the human is not on the set, then bring in the human & remove the ghost & then fire the flash on the human. There are several ways that you can try that would produce several outcomes depending on your exact needs, but for this set up just get the ghost to move slowly, then bring in the human fast enough so they don't show, especially if you want the ghost to appear as if it's passing behind the human, & then trigger the flash when the ghost is out of the frame.