During an interview with io9, the director promised there would be no CGI in his movie, a decision that came with a price:
"We didn’t do any CGI in the movie. There’s no CGI in the movie. Everything you will see is real, which was really demanding. This was a very long shoot, 70 days of shooting at night. There’s a reason people use CGI it’s cheaper and faster, I hate that. We researched a lot of magic tricks and illusion tricks. [Like] how you would make someone’s arm disappear."
This is amusing, considering Alvarez's break-out short film was almost exclusively CGI (check it out here), but with greater resources comes greater ambition and I admire the director for opting to use an increasingly forgotten art form. Too many horror movies now opt for what is easy at the expense of what makes an impact. Nothing quite has the impact as some practical blood and gore effects, there's a texture and tangibility that just cannot be emulated by CGI. Based on the exceedingly messy trailer, I can see so much texture that you're practically slipping in it.