As far as special effects and Halloween-style horror goes, there are few things more effective than fake blood. I love using blood squibs for costumes and stage effects, so I may be a little biased here, but I do think it’s one of the easiest ways to make any look inherently more horrific. There's something very visceral about the look of blood that's hard to replicate. Whether you’re dressed as a murderous animatronic, a serial killer security guard, or a zombified purple guy, a little bit of fake blood can go a long way. Of course, a lot of fake blood can go a long way, too. How much gore you add to your costume is up to you. If you want to push your makeup skills with your FNAF costume, however, special effects makeup products like liquid latex, fake wounds, and more are typically available at Halloween and party stores as well. While these require a bit more skill to handle and blend, they’ll give you a lot more options for creating costumes, skin-peeling effects, and fake wounds that look really authentic. A peel-away liquid latex effect would work especially well for characters that transform from human to animatronic in the series, such as Circus Baby, Michael Afton, and William Afton. While you can achieve similar looks with nothing but makeup, latex and spirit gum add a lot of depth and detail that can’t be replicated without them.