Agents frequently use possession to move quickly from one location to another. Though the process only takes a brief time, the Agent is vulnerable to attack during this time. Outside of killing an Agent's shell in battle, killing the bluepill during this brief vulnerability amid possession is the safest known method for any Resistance member to escape an Agent.
While possessed, bluepills are virtually placed into a coma, having no awareness of any events during the possession. Bluepills left alive after a possession are simply confused after returning to consciousness, sometimes badly injured.
Should an avatar be killed while possessed, only the bluepill originally owning the avatar dies, leaving the corpse to revert to its original RSI. The Agent previously possessing the avatar simply jumps to possess another body.
After resisting deletion, Agent Smith began to use a method of possession that may have been derived from his old Agent ability to possess bluepills. His method was much more direct; a Smith avatar would force its hand into the RSI of his intended victim, and release a black substance that would slowly overtake the victim RSI and, once completely covered, convert the avatar into another Smith.
There were a number of differences between normal possession and Smith's version:
- Smith's version was much slower to act (possibly because he no longer had direct access to the Matrix itself) and was also reversible as long as the victim RSI hasn't been completely covered by the substance.
- Smith could not remotely possess an avatar like normal Agents; he had to be within touching distance to be able to infect other avatars.
- While normal Agent possession can only affect bluepills, Smith's version can affect any and all beings inside the Matrix; other Agents, Exiles, and even redpills. Given enough time and the connection between the host body in the Real World the avatar in the Matrix, Smith was also able to jack out of the Matrix, leading to Smith possessing a human's body.
- A Smith that possesses an Exile with a specific function can utilize that function itself, and may also take on memories and traits of the victim Exile, as seen with the The Oracle's Smith.
Neo is the only one known to have successfully killed an Agent by making direct contact and merging with an actively possessed avatar. He then effectively tore down the Agent from the inside, ultimately sending the occupying Agent directly to the Source and thereby erasing the program.
Likewise, Neo's Smith-infected avatar was used to stage the cascading elimination of all the Smith virus after Smith had already taken control of the entire Matrix. It is assumed that this was only possible because of Neo being The One and being the Prime Program.
While never clearly stated, some Exiles may actually have the ability to possess bluepills or other programs and that other programs have the ability to change their shells. This was seen when The Oracle used a new body after The Merovingian used kill-codes, which he acquired in exchange for Sati's safe transport into the Matrix, to target and destroy her old shell. It is presumed that The Oracle changed or "shed" her shell, thereby escaping to a new one before the old shell was destroyed by the kill-codes.