Most directors and producers want to make their movies as realistic as possible. Whether making a gritty action film, romantic comedy, or terrifying horror movie, they want audiences to believe what they're seeing is real. Such credibility comes from many factors, and requires the performances, script, and other facets to be top notch, to ensure viewers don't lose interest. One of the biggest aspects of maintaining believability in a theatrical release is special effects; if a film can't replicate what people see in daily life, or create credible alternate realities, that movie quickly lose authority.
Although people generally think of special effects as explosions and CGI, the truth is, prop designers and FX experts are involved in the effects process throughout the making of a movie. They especially come in handy when the filmmakers need a nasty substance. After all, you can't expect your cast to actually bleed, or wade through raw sewage (unless you're Neill Blomkamp), even if the script says just that. So the effects team has to come up with safe alternatives. A lot ingenuity goes into gross things used as props in films, especially things used for gross substances in movies (puke, for instance).
If you've ever wondered "what do they use as blood in movies?" or how filmmakers recreate vomit, snot, sweat, slime, ejaculate, and more, read on: you’ll be amazed by the creativity and resourcefulness of those responsible for creating special effects.
K-Y Jelly is a water-soluble substance most often used as sex lubricant. It's also a staple of horror and action movies, thanks to its sticky, slimy look. Almost any film with a monster covered in a vile or slimy substance uses K-Y to achieve the effect, as demonstrated most famously in Alien, for which lube was used to cover the alien, and create its signature thick saliva. According to Turner Classic Movies, screenwriter Dan O'Bannon was reminded of "a horrific sexual experience he'd once had" while visiting the set. Other prominent films to use K-Y include The Thing, Jurassic Park, and Predator.
At least, most of it is. Filmmakers seem to have used everything they could get their hands on to make the right type of blood for any given production. Whether it was Alfred Hitchcock using Bosco chocolate syrup in Psycho or tomato ketchup in Scary Movie, anything vaguely resembling blood has been used on screen. A mixture of corn syrup, food coloring, and methyl preservative is the go-to for most, though, as it can last a long time between takes and looks incredibly realistic. This recipe was concocted by Dick Smith, whose work on The Godfather, Taxi Driver, and The Exorcist revolutionized the aesthetic of bloodletting in cinema.
With all that corn syrup, fake blood can get pretty sticky. While filming Evil Dead, Bruce Campbell's shirt reportedly became so stiff with fake blood it broke, while Sissy Spacek said she felt like a candy apple while covered in blood in a burning room for Carrie.
Not every movie blows its load with a money shot, but those that do need a convincing substitute for semen (There's Something About Mary, Happiness). Methyl Cellulose is the most commonly used substance in the mainstream and porn film industries. Its use arose in the porn industry to fake semen in women's nether cavities, for which filmmakers needed a realistic looking substitute for jizz that wouldn't cause yeast infections or other health problems. In addition to being used as fake ejaculate, the gooey gel has been used as slime in Ghostbusters, lava in Volcano and Star Wars: Episode III - Revenge of the Sith, and molten metal in Terminator 2.
For Y Tu Mama Tambien, which features a scene of two teenage boys masturbating on diving boards and shooting their loads into a swimming pool, filmmaker Alfonso Cuaron and his team used shampoo. They bought it, squirted it into the water, got their shot, and moved on. Easy peasy.
It’s pretty obvious no one, not even struggling actors, wants to be covered in snot. This means special effects teams have to concoct a believable substitute for nose semen whenever a script calls for it (maybe this is more on screenwriters than anyone else. what's wrong with these people?). A common solution is a mixture of common detergent and cosmetic ingredient borax, Elmer’s glue and water. Unfortunately, this mixture is very sticky, and can cause problems for actors, as Martin Freeman made clear when talking about The Hobbit.
"It's not a lot of fun to spend days and days in gelatinous snot make-up. Because of continuity, you've got to be in wet snot and then the remnants of drying snot. That's a lot of days. It's kind of like a hair gel. Sticky. Very cold. Not comfortable," he explained.