Maybe someone can help me with shadow casting on vertical planes. 
If I put a character in front of a plane which is running a video, and I am try to insert my character in the video scene as realistically as possible,. I would of course like the shadows to function as if I was working against a floor plane. I've spent hours with the spot- and directional casters and put shadow catchers in all sorts of positions and angles but when my character moves the shadows just dont follow in the desired manner. 

I get sufficient results as long as my character is standing still - but not when movement is involved.
Since characters can't easily be scaled, I move them on the both the X, Y and Z  axes to get them deeper into the video scene and that's when the difficulties occur. Can anyone help me. 

You should not move the character. Move the plane instead. And character can be easily scaled to at least 50% without much disturbance (depends on a character and outfit). Character Body Scale is right at the top of the modify panel next to Auto-Blink.
If your iClone camera and camera in the video are static, then there should not be any problem with shadows and tracking as long as character is fairly far away from the plane.
Video also got to have sufficiently high resolution and be scaled respectively with plane to accommodate the character. Just need to position camera properly.
If your iClone camera is also moving/rotating, then you have much more work with tracking (moving a plane and shadow angle to compensate). Hard, but doable to acceptable level.

Nothing fancy...

Setup



Subtle camera rotation/movement compensated by a few video plane transform points for tracking (linear transition curve presets for both camera and plane keys).
Tip: Ctrl+Q to avoid Gizmo and move plane with mouse.
Default iClone lights slightly customized. Default floor shadow.
Base girl avatar without scaling at world center.

Outcome is not perfect but quite acceptable (still don't understand how she managed to dance on a sand in heels :w00t:)

Excellent solution, as always, Gurgen. :):cool::smooooth:

Gerry

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System: Win 10 Pro (21H2), Asus X99-E WS, CPU i7-5930K -3,5 GHz, 32 GB DDR4 2666-16 RAM, NVidia GTX 1080 Ti GPU - 11 GB VRAM(Driver Studio-536.99), Samsung 850 Pro 512 GB SSD, 6 TB HD storage.

Hats off for such a detailed explanation. I think one of the complications for me on the this production was that the video was heavily backlit. I need the shadow to be in front of the character. Would this make a difference in your opinion?

@wires,
Thanks Gerry :)

@mindprints

You can have a shadow at any one of 360 degrees angle. The lights/illumination have to be setup differently though.

Setup lights and shadows. As usual, better start with totally dark scene (IBL, lights, ambient, GI - all are off or 0). Do not worry about video background being dark.



Then enable GI and add Ambient light (color other than black).
Enabling GI also allows adding emissive surfaces, objects. Any prop can start emitting a light if enabled in GI section of the prop material. Infinite possibilities are here.



Now, in Video Plane Materials crank a Self Illumination to brighten it up.




Optionally play with LUT effects (or build your own with Color Adjustment/Filters) for desired artistic look.

Cool. Unfortunately I have to produce one video a day and gave up on my example project. I released it without shadows. Sadly.  I have been testing your ideas. ended up with the shadow upside down:-( and gave up, but there will be plenty of more chances. Your detailed help makes this forum worthwhile. thanks.  

Greg

Oh, thanks, next time then. Good Luck :)

For anybody who confronts the problems we have discussed here is my video. Due to time constraints (I make similar productions daily) I was not able to workout the shadows. Some people will never notice, but I do, as will Gurgen. Here are my comments:

1. I could not scale dicho's koala bear. If you can't scale a character that creates big problems for simulating distance from the camera. 
2. I didn't have my camera on a tripod. You can see how the bear seems to float in sections of the film since the background isn't steady. 
3. I missed shading the bear when he passes in the shadow of the two boy skaters. That would have been a nice touch.
4. I didn't have time to create some sort of blur effect on the ice marking the bear skidding. This could have  been done in my video editor. Or I could have done some comical smoke with particle emitters.  No time for that.
5. And finally - no shadows, which this discussion has been all about. 
 
I hope somebody learns from my mistakes/omissions    

1. You should be able to scale any character. Looks like you have non standard. So you should use Avatar Proportion



There is a 50% limit on scaling down. For something smaller, avatar has to be scaled outside like in Blender.





Or like here. Cops and a car are really tiny:



2. It's barely noticeable since the scene is on the ice. No one would pay attention. But if you want to perfect it, you should animate a video plane to compensate and track. Might be hard depending on how jerky camera was.

3,4. Yes, that would have definitely been a nice touch :)

5. Oh well, don't know why. If I had that project I could have helped to figure out.