Creating Animated Video Mask

Get the trial of After Effects and use the rotobrush it will be the fastest way to do it. But if you want to do it in Photoshop then you can only use the clipping mask on an image layer but not on the Video Layer so here are your choices.

Paint on a Separate Layer and use this as your background.

1. In the Timeline or Layers panel, select the video layer.
2. Move the current time indicator to the video frame you want to edit.
3. choose Layers > Video Layers > New Blank Video Layer.
4. Painting on a blank video layer is nondestructive. Select the brush tool that you want to use or use a selection tool like the quick selection tool to select the area you want to paint on (I selected the guy then inversed it and filled the new selected area)
   
   
   
5. Go to the next frame and repeat. (The rotobrush tool in After Effects uses motion tracking, optical flow, and various other techniques to propagate the information from the base frame to the current frame to determine where to draw the segmentation boundary. So it does a lot of this work for you. In Photoshop you are doing each frame by hand)
6. Add other Video Blank Frames to animate your background elements. (these items can use the new

Rotoscope it all in Photoshop

1. Choose FIle> Import> Video Frames to Layers
2. Choose your options.
   
3. Each frame becomes a frame in the Frame Animation Timeline. Edit each frame.
   
4. Export.

Let me know how you make out. AE is your best bet!

Yes, I agree it would be more efficient to do this in After Effects but you can do it in Photoshop with the use of animated Clipping Masks. Here are the steps:

1. Open the Timeline panel via Windows> Timeline. Then click on "Create Video Timeline"
2. Have two layers: Top one for the video or image and the one below as the mask
3. 
4. Choose the image layer and go to the menu Layer> Create Clipping Mask
5. Position the mask layer where you like it then in your Timeline choose the Position stopwatch to create your first keyframe.
6. Move down the timeline and move the mask layer to create your second keyframe. Between the two keyframes you will see your animation.

In After Effects it would be best to use a Track Matte though the Photoshop does go into After Effects as an Opacity mask.

What Mr. Pfaffenbichler said - nothing you can do in PS. This requires a proper VFX compositing/ motion graphics tool like AE.

Mylenium

It sounds like a bad idea to do this in Photoshop (even if it should be possible with Blend If-settings for example), if you are at all serious about video/film editing you may want to consider using After Effects.