How to render casted shadow on surface only How can i render a character + the shadow it cast on a surface but not the surface itself?

I would use it to 


Render my background only once (if nothing moves in background) so i would be saving a lot of time 
Better effects with transparency gif
Cycles Render
Select the surface object, go to Object / Cycles settings and check Shadow Catcher.
In Render / Film, check Transparent.
Blender Render
Select the surface object, go to Material / Shadow, check Shadows Only.
In Render / Shading / Alpha, select Transparent.