Once a Head crab successfully attaches to a host, the Headcrab will assume control of all their motor functions, able to walk and attack with the host's arms and legs. Through some unknown biological process, the host's hands turn into long, skeletal claws and a massive hole appears in their chest lined with the hosts' ribs, becoming a makeshift "mouth", with their ribs serving as teeth (zombies have been observed to feed by ripping chunks of flesh off of corpses and stuffing them into this maw) Removal of the Headcrab reveals that the host's head is bent abnormally upwards, their hair is matted with blood, and their face is pale, with their eyes shut and mouth open in a frozen scream. In Half-Life 2, the muffled pleas and screams coming from the zombies indicates that the victim may be somewhat aware of their zombified state and obviously in an unimaginable amount of pain the entire time. Occasionally, Headcrab zombies may survive losing both legs and their lower torso, and continue to crawl along using their arms. In most cases, a Headcrab remains attached to its host until the zombie is destroyed, although in Half-Life 2, they will occasionally detach from the host if it is rendered immobile by its injuries.
- When a zombie lunges at an opponent in all games of the series, their arm appears to stretch out slightly when they swing. This could just be a graphical embellishment, however.
- In Half-Life and its expansions, standard zombies have a (mostly) human walk. They also have their "claws" up in what could be considered as a "battle-ready position." In Half-Life 2 and its episodes, the standard zombie walk has become a slow, awkward shamble, not unlike that of zombies in the movies. Their claws are also seen being held up closely against their bodies.
- Playing parts of a Half-Life 2 zombie's sound files backwards will produce extremely disturbing yells of pain. The reversed cries have been interpreted as "Oh, God help! Help me!" and "Get it off me!" If set on fire, the victims may say things that might be interpreted as "Why, why, why!?", "Help, God... help!
- The screams of the Half-Life 2 zombies are heard muffled in-game. Speeding up the sound file to the screams shows that the muffled effect is caused by the slowing down of the file.
- In the non-HD variant of Half-Life and its expansions, a zombie's headcrab appears to be fastened tighter to the victim's face, revealing the eerie outline of a human skull.
- Two Standard Zombie growls can be heard in the film Drag Me to Hell. The Half-Life series uses many stock sounds heard in many other video games and films.
- In Half-Life: Opposing Force, spawning the dead soldier zombie will reveal its pose as that of spawned deadHECU grunt.
- In the 1998 version of the Half-Life Prima Guide, Headcrab Zombies are referred to as "Mawmen", most likely because of the mouth made in their chest out of their ribcage.
- The security guard and HECU zombie models seen in Opposing Force have no HD model.
- In the book The Zombie Recognition Guide, the Head Crab Zombies appear under the name of Crabheads and are mentioned to have come to earth in a meteor.