Video: Adam Savage and the Tested crew recently had the opportunity to visit the sets of Alien: Covenant, and in their latest video they not only get some hands-on time with many of the weapons used in the upcoming prequel, they also learned the secrets of how alien-infected actors are able to vomit so much blood.
You probably assumed the actor simply held the fake blood in their mouths until the cameras started rolling, but that limits how much they can spray and spit out. Hiding a hose in their hands which they have to hold to their mouths isn't that convincing, and adding it all via computer graphics in post-production is pricey, and makes it harder for the performer to react appropriately on set.
To really sell the idea of a character's insides being torn to shreds, the special effects crew on Alien: Covenant used a polished brass tube, with several strategic bends, that hooks around the side of an actor's mouth, opposite the camera.
The fake blood (at least the food-safe kind) is pumped into their mouths through this brass tube, which is connected to a longer hose that remains hidden out of sight. The actor simply has to simply keep spitting out the blood as it fills their mouth, without looking directly at the camera while they flail around in pretend agony, which would make the tube visible.