Different solutions to a character in an AGS game in front and behind something!

[TheManInBoots]

then select the counter and create a mask from that selection [...] and apply it to the folder with all the sprites that should be affected, so that any part of the sprite that is behind the counter will not be drawn

What exactly do you mean with "all the sprites"?

Well, personally I would do it as well with different characters, but that's a personal preference.

But if you look for simplicity -if that's what ManicMatt asked for-, and you have only one sitting position, then as Ali pointed out, pre-masking the character is the easiest solution,
because you do not have to create a second character and animation.
If you have the arms separately in your image editor, you can pre-mask the character sprite using the table mask as you described, and just layer the arms on top of the cut out character.
So that eventually you have only one single animation and character for the table animation.

[ManicMatt]

For my second character that is much further along behind the counter, I don't intend for them to move around as much as the other character here, and will be rather static comparebly, so I have done as Ali demonstrated.

Oh yes, I always draw arms, legs, head and torso (and sometimes hips) in different layers, which is something I highly recommend if anyone isn't using layers already.  It's especially useful when it comes to animation. Although I do on occasion start drawing on the wrong layer, doh!

[TheManInBoots]

Although I do on occasion start drawing on the wrong layer, doh!

Yeah, happens to me too sometimes! When I click to make a layer visible and believe it's also selected because of it, when it's actually not