From Battlestar Wiki, the free, open content Battlestar Galactica encyclopedia and episode guide
The Cylon Centurion is a fully armored, metallic automaton that functions as the basic foot-soldier of the Cylon ground forces.
Modern Centurions serve a variety of functions within the Cylon hierarchy, acting as guards, soldiers, executioners and even gardeners. "Bullethead", "clanker" and "chrome job" are common Colonial nicknames for these advanced and deadly Cylon constructs ("Downloaded", "Rapture", Battlestar Galactica: The Resistance). They are also referred to as "toasters" - the common Colonial nickname (or epithet) for the Cylon race as a whole (Miniseries, "Resistance").
Although Centurions are sentient by design, an implant annuls their higher cognitive functions and prevents them from exercising full self-awareness ("Six of One"). It also appears that this implant precludes them from harming any of the humanoid Cylons, except by explicit order ("Hero"). According to Sharon Agathon, this was done to prevent a rebellion akin to the one that started the first Cylon War (Razor).
Following the Second Attack on the The Twelve Colonies, Centurions were used in a number of assaults on the surviving human Fleet. Centurion forces boarded both remaining Colonial Battlestars Galactica and Pegasus ("Valley of Darkness", Razor) and landed ground forces on Kobol and the Algae planet ("Fragged", "Home, Part I" and "II"). During the brief colonization of New Caprica, Cylon Centurions were used to keep order throughout the Cylon occupation ("Lay Down Your Burdens, Part II", "Occupation", "Exodus, Part I" & "II"). On Caprica, modern Centurions were part of a clean up effort in Colonial cities including Delphi where they participated in the reconstruction effort ("Downloaded").
Centurions are unable to distinguish one copy of humanoid Cylon from another ("Precipice"). Whether Centurions are able to recognize members of the Final Five - like Raiders are able to - is unknown. It should be noted, however, that prior to the activation of said models, Centurions directly engaged at least two of these individuals in combat ("The Farm", "Fragged").
During the Cylon Civil War, rebel Cylon leader Natalie removed the telencephalic implant of the Centurions under her command. Once the device was removed, the Centurions gained the ability to reason for themselves and subsequently turned against the rival humanoid models ("Six of One").
Self-aware Centurions operating aboard the Rebel Cylon basestar within the human fleet, cooperated with Colonial forces to rescue Hera from the Cylon Colony. Distinguished by red stripes painted across their armour, these Centurions operated as assault troops under the command of Lee Adama. During the conflict, "Red Stripes" battled other modern Centurions as well as older models living on the Colony since the first Cylon War. Following the destruction of the Colony and the discovery of "Earth", the Centurions were given command of the Rebel basestar and set free by Ellen Tigh and Admiral William Adama. Despite the risk that they might someday return and attack humanity, both Tigh and Adama noted that the Centurions had earned their freedom ("Daybreak, Part II").
Modern Cylon Centurions are at least mostly robotic beings in nature. Unlike their Cylon War-era predecessors, the current models have not been observed "bleeding" when injured ("33", Razor, "Daybreak, Part II"). More agile than those predecessors, modern Centurions are capable of extremely fast bipedal locomotion ("Six Degrees of Separation", "Tigh Me Up, Tigh Me Down", "Daybreak, Part II").
Sleeker in appearance than their robotic ancestors, Centurions are humanoid in structure, with long limbs and heads partially encased in armor. Their hands have four long, multi-jointed claws that can be retracted into smooth-tipped fingers of human proportions to handle more delicate objects ("Valley of Darkness", "No Exit").
It is unknown whether Centurions can "speak" to other Cylons - though they likely communicate with each other via a wireless network, making verbal speech unnecessary. They can, however, hear and respond to verbal commands from the humanoid Cylons ("Hero").
Some versions of the Centurion wield markedly improved armor. A platoon of Centurions that boarded Galactica were unaffected by standard weaponry; subdued only by armor-piercing explosive rounds ("Valley of Darkness"). Other Centurions appeared to be constructed from an alloy that is much lighter than that used in earlier models. During combat aboard the Colony, loyalist Centurions of the newer type were cut down by Colonial fire more forcefully than the earlier 0005 models ("Daybreak, Part II").
The Centurions that Helo encountered on Caprica reacted more slowly, behaved less intelligently, and were more easily destroyed by small-arms fire. However, if one takes into consideration the overall plan that the Cylons had for Helo and Sharon Agathon, it may be surmised that these variants were intended to inflict psychological, rather than physical harm. Conversely, Centurions seen on Kobol displayed an increased group intelligence, working together to construct an anti-aircraft battery from parts of their Heavy Raider ("Fragged").
The modern Centurion can effectively oppose a squad of Colonial soldiers by means of various combat systems. Long, talon-like fingers act as bladed weapons during close-combat, and fold back to reveal a three-barreled automatic projectile weapon which may be used in fire-fights.
In addition to these built-in weapons, Centurions are capable of carrying larger, more powerful weapons not integrated into their systems, such as mortars and missile launchers. In "Razor", one Centurion boarding Pegasus carries a large, and likely powerful, machine gun in its hands.
The creators of the re-imagined series decided that using actors in an elaborate robot outfit (as was done in the Original Series) would detract from the realism of the show. As such, the production made use of computer-generated Centurions in place of actors.
Inaugural director Michael Rymer originally wanted the Centurion to blend into the background (so as not to detract from the live-actor performances), but its design proved eye-catching, and the robot became a central addition to many scenes. These virtual Cylons were designed with full articulation in the hands and joints for realism, and were initially animated with more human movements, such as swaggering. However, these movements did not complement the robotic attitude that the writers wanted to convey, and were later discarded.
When filming a scene that will later have virtual Cylon Centurions inserted, a life-size cardboard stand-in is used for framing purposes. The stand-in gives the camera operator an idea of how much space to leave in the frame, and the animators a reference on how big the Centurions should be. The stand-in is removed so when shooting begins, the actors are reacting to an empty space. This minimizes the amount of work VFX must do to add a Centurion to the shot.
As of "Faith", a new model has been used to depict the Centurion. The new version features a bulkier head with sleeker lines and significantly sharpened cheek bones, separated collar and shoulder pieces, five-fingered (rather than four-taloned) hands, as well as an increased level of detail and flexibility over the entirety of its body. In the episode "No Exit," a Centurion using the new model had more detailed hands in a few close-up shots which were based off the original model, rather than the redesign. Though originally considered a retcon of the older model, there is evidence that this may not be the case. The previous design can be seen once more during the battle of the Colony, occasionally in the same shot as the newer variant. Generally, the Rebel Centurions were represented by the new design, while Cavil's Centurions used the original model (Daybreak, Part II).