[Sascha Reisüber]

Thanks for all the responses. You´ve helped me a lot. The idea from Joey was great.
The effect which I get, if I work with Luma Matte, is precisely what I need.

Further I am testing some effects with lights out, so that the guy doesn´t look like someone who is glued in the scene.

@Dave: I want to set the guy in a medium close-up, so that you only see upper part of the body.
You are right if you say that the situation can´t work in the real world. But to quote Richard Willams (Who framed Roger Rabbit), the situation don´t need be realistic, it must be believable.

Thanks for help everyone!