A green screen is great, it is used behind actors to put them in different places, as long as the background is a solid green or blue colour and you dont wear the same colour yourself you can do what they call 'KEYING' out using CHROMAKEY.
What is green screen?
Some effects you can do when you are
filming, this is the best way as it looks good and you will save lots of edit time.... for example
Making a fountain effect
How about cloning yourself or playing lots of characters in the same scene..... here's how with a bit movie magic...
here's more camera tricks to get some interesting shots like finding something or filming through a keyhole... get creative!
FILMING THE KEYHOLE
HOW TO EDIT WITH GS
CLONING WITHOUT GS
CLOninG WITH GS
or how about starting to fade away going transparent, disappearing completely ... have fun trying these out in your films!