Hi FoxySeta, we already got a few people who wished to calculate a character by variable too, so we are working on that. Right now, only Show Picture's File Name can be calculated by variable in all cases. The problem with characters is that they are associated with a character expression which is associated with one or more image files.
So if you calculate a character by variable, the engine has no chance to preload the character expression assets because it is only known at runtime which character should join. In Show Picture, as a first experiment, we implemented it in a way that we pause/freeze the entire game and game loop until the asset has been loaded. It seems to work fine so maybe we will apply the same approach for Join Scene in the future.
So Join Scene by variable only works if the asset of the character already has been preloaded somewhere. So I can see two solutions here:
1. Use Join Scene only for the character and Show Picture or Show Animation for the expression.
2. At the beginning of your game, place a bunch of Show Picture commands, which all intending/shifted to the right showing all your character's images. With that, since the commands are shifted they are not executed BUT the preloaded will see that and preloads all your character's images at game start. Then Join Scene by variable just works because everything has been preloaded already. But that only works well if the amount of character assets is small, otherwise it would take too much memory.
Let me know if that helps you out.