invisable object

I’m adding an animation to a home video. I want the model to move behind a object in the 2d video. I was thinking I need to have a object the camera cannot see but blocks the second object from the camera as it moves behind, I was thinking premul but I will see both objects. Or maybe green screen .

is the home video still, or does it move around?

not a video

Video I did it last night with green screen When i get the green from the fish It also removes the shadow. [video].[/video] Still not solved . I like to find a way to skip the green screen. But this is what I working for. Thanks
also how do you get the vimo video to display here?