The appearance and behavior of a Stalker differ a lot from those of a human, being bald, lacking a nose, and having grey skin. Apparently unable to talk, it will simply roar and make groans if bothered. It has small plates on hinges next to its eyes, which can be apparently closed during periods of inactivity. Stalkers also have the ability to emit a laser beam using the apparatus mounted on their head; they use this tool to repair Combine devices or in self-defense; it is also capable of destroying Energy Balls.
Stalkers' limb extremities - hands and lower legs - have been removed and replaced with bolted-on metallic augmentations which offer reduced agility. For ease of maintenance and to bring a dependence on the Combine, the Stalker's digestive tract, as well as the majority of organs, has been taken out, resulting in it surviving solely on a nutrient solution that the Combine provides, as evidenced by the gastric port in their sides. Stalker's body is heavily atrophied, with very little muscle or fat tissue remaining. All signs of its genitals have been surgically removed. Stalkers also appear to be resistant or immune to radiation, as they are seemingly unaffected even when working near the extremely radioactive Citadel Core.
The Stalker transformation process is performed at Nova Prospekt, where Combine scientists sever higher brain functions, experiment on the lobotomized victim in any number of foul and depraved ways, embed a number of blackened nano-devices in the skin, and slowly watch the victim lose all humanity. After this cruel process, they are brought to the Citadel, where they are required to maintain the core and guard from intruders. They are transported by Razor Trains, confined in Prisoner Pods.
Despite being seen in the Citadel, Stalkers are never actually engaged in combat during Half-Life 2. In Half-Life 2: Episode One, however, it is possible, and sometimes - required, to engage in combat with them in order to more easily achieve mission objectives, as their lasers prevent certain tasks from being completed. Episode One is also where the exact nature of Stalkers is first revealed, as they can be seen up close. Stalkers aren't aggressive - they don't attack Gordon, unless another individual is killed right in front of their eyes, but instead walk away as if he wasn't there at all; as stated by Alyx, Stalkers "shouldn't bother us if we leave them alone".
It appears that the Resistance members have a full knowledge and understanding about Stalkers, what they are, and how and where they are made. Alyx Vance comments on Nova Prospekt: "It used to be a high-security prison, it's something... much worse than that now." Barney Calhoun also makes the comment "He was about to board the express to Nova Prospekt!" at the start of the game, and sometimes Resistance members say, "They're not gonna make a Stalker out of me".
|Half-Life 2: Episode One|
|Contain the Citadel core without killing any Stalkers.|
Behind the scenes
According to Ted Backman, the enemy's designer, the Stalker was inspired by a black version of the Half-Life Skeleton multiplayer model. The developer who created and featured it in his levels proposed that a "black skeletal character that can hide in dark shadows and leap out at you as you got close" be included in Half-Life 2. The horror aspects of this concept were then further developed; the team wanted to have "something that crept around in the shadows and then lunged" at the player. However, as explained by Backman, they "took a half-baked idea and turned it into something more horrific because the Stalkers are really the victims. When you face them, they are these crazed half-human things that you can't help but almost pity. I was hoping to put enough humanity into the things that it wasn't just a scary monster. Instead it was something that presented a moral dilemma every time you had to deal with it, which I think is a more interesting problem. It is more horrific to have to deal with an insane hostage than something that just wants to eat your brains."
Marc Laidlaw expanded on the enemy's origins, saying that Stalkers were one of the earliest creatures put into Half-Life 2, and the developers tried them in several gameplay environments. Initially, they were something that crouched down in the dark and followed the player from behind; he would turn around and see them. Valve had all kinds of gameplay, but they turned out to be devoid of fun. The developers used the Stalkers in puzzles where they could cut through sheet metal with their beams but since the sequences were scripted and too stagy, they ended up aiming for an emotional quality with them to make them an important story element.
The Stalker was originally planned to be fought in Half-Life 2, but this feature was cut and, later, re-introduced in Episode One. The Stalkers in the Half-Life 2 leak had melee attacks, their laser attack did more damage and they would emit a high pitched scream when killed.
According to Bill Fletcher, Alyx's reactions to the Stalkers in the crashed train in Episode One went through a number of iterations. "In very early versions, Alyx was frightened by the Stalkers. That didn't work because it didn't fit with Alyx's earlier reactions to Stalkers in the Citadel. We changed it so that she expressed anger at the Stalkers' predicament. Early versions of this new direction failed because Alyx's transition from generally upbeat to blind rage was simply too abrupt. Ultimately, through rewrites and rigorous testing, we found the right tone for the scene - a simmering anger level that was more in tune with the situation."
A Stalker inside the early brush Depot.
A stalker being supervised by a Combine Soldier in the Citadel.