Design, VFX supervision.
I was in charge of creating the film damage for this film. Robert Rodriguez wanted the film to feel like it had been cobbled together from pieces found in the basements of derelict film houses, damaged by years of neglect, leaking roofs, kids with razor blades, etc. Moreover, he wanted the damage to be a bone-fide character in the film.
We spent months studying film damage: how it gets damaged, why, and what it looks like. Then we built a procedural machine which we used to dial the film damage with sliders -- much like sound design -- across thousands of shots. Since the film damage spanned the entire film, it was also perhaps the longest VFX shot ever!