I used a projector, so my image was done on the computer first, and then projected onto the wall and then traced, i traced where i saw shadowing, the only way you can realy make shadowing look right is to blend paints til you get the right tone, but drawing it on the wall WHERE it needed shaded made it easier. Yeah, I was definitely going to say to prime it first, I used white primer, it took 3 coats to cover the horrible purple my room originally was...
I painted the base color after that, but my background wasn't complex, just grey, I do know i had issues with the moon and Jack, and the only advice I really have for that, is patience and a SMALL paintbrush for the edges with a nice pointy tip. You want it to be clear where the character starts and where the background ends, and you dont want either spilling onto one another, you will have to do some touch up before it is perfect.. Luckily for me, NBC wasn't ever PERFECTLY drawn, which gave me SOME slack but not a lot.
Youre not doing stripes so the texture isnt a big deal, but if you do any lines that you're taping off, since there is texture, theres a technique to seal the tape before you paint them, so that they come out crisp.
As for paint, And what to use, If the wall is exposed to a lot of sunlight, everything I read online said to use expensive high quality acrylic. Since my wall isn't exposed to sunlight, I surprisingly just used the cheap acrylics you get at the art store in plastic containers, after they are painted, you need varnish in whatever finish your paint is, mine was Satin so I got satin varnish, its shininess matches the wall and it is protected, also.
I hiiiighly recommend drawing out your design either on paper first or the computer and getting it projected onto the wall. My projector was a cheap crappy one from Walmart, but it did what I needed and it worked pretty well for the purpose.