Well usually I try to have a simple background to have a good focus on the subject, sometimes I put a pattern in there to signify something, usually emotion,

or in this case I was trying to signify that this particular panel is a quick flashback to the 70s, so I gave it a sort of 70s hippie style
I usually add it when I want to convey either something to do with the story, or to display something to reveal about Emerald (seeing as so much is set inside her house) i.E.how much she loves birds. After all, you can see how much random bird stuff is in and around her house.



Or when I am doing a Family Guy establishing shot
Or feel like drawing some weird shit

Or a background gag

Or the times I do this cross-sectional isometric view that I like to do

And then, of course there are the times where the background details change up mainly because I am doing a panel or two in a different art style for emphasis




(yes, I have an actual painting I did on canvas with cheap acrylic paint for a panel)
And then, lastly, I would like to point out that ever since chapter 12 (and as a trial on chapters 6 and 7), a thing I have been doing is, in the white space, I have been adding patterns and gradients as a sort of second background for entire chapters/scenes
They can range from relatively simple

To kind of complex
