I'd shoot a clean plate of the shot with the formation on the ground minus the character. Then shoot with a green screen your character as he appears and interacts with anyone else in that shot. Then make a third plate of just effects such as smoke. In after effects use a combination of your smoke, particle world/playground. You can finish the effect of making the character appear by using it in addition to a luma matte of perhaps more layers of smoke. Selling it may require adding a little camera movement in post or outside things occurring i.e. flashes of light, leaves being blown around. All things to think about.

Video Copilot also has a tutorial that may help you go even deeper: http://www.videocopilot.net/tutorials/the_portal/

I would love to see the finished product!

Thanks for sharing!

What a cool unique story. Thanks for sharing!

To answer the guy on the street question: Post a clear and highly visible public notice around the area you are filming stating by entering the area they are giving permission for you shoot them. Take photos of where you clearly posted the notices in someone tries to say you didn't post anything. Always go on the side of caution and try not to feature someone who is not a part of your film. And if someone comes back and asks to not be included, be courteous do another take.

Just found this. Thanks, this is awesome!

Best of luck to you getting funding for your first short film. While possible, its unlikely you'll get the money you are looking for. If this film is truly that important then either find someone whose more established that can help you bring it to life or pocket this film and and write something else to work on building your audience to help fund your next film. Show them when you can do with a zero dollar budget.