The walkbehind allows you to draw over an area of the background and set a baseline. Sprites will appear either in front of or behind that area depending on their Y position in relation to the baseline. That area can be whatever shape you draw.
Objects, like characters are sprites with their own baseline (the bottom of the sprite, by default). You can't draw walkbehinds on sprites. So as long as the baseline of the desk object or desk walkbehind is below the character sprite, we shouldn't see their legs.when they're behind it.
If you want a sprite that is partly in front of and partly behind the same object, or walkbehind - that's more of a challenge. There are a few solutions, discussed recently in this thread: https://www.adventuregamestudio.co.uk/forums/index.php?topic=57953.0
Objects, like characters are sprites with their own baseline (the bottom of the sprite, by default). You can't draw walkbehinds on sprites. So as long as the baseline of the desk object or desk walkbehind is below the character sprite, we shouldn't see their legs.when they're behind it.
If you want a sprite that is partly in front of and partly behind the same object, or walkbehind - that's more of a challenge. There are a few solutions, discussed recently in this thread: https://www.adventuregamestudio.co.uk/forums/index.php?topic=57953.0