OOC; ABILITIES OF THE GEASS.
* The user can only use it once on a given individual. Any following attempts will be ineffective.
* The victim is bound by his or her physical and mental capabilities. Asking the victim about or to do something that he or she doesn't know or cannot do will produce no effect.
* Indirect eye contact via a reflective surface, i.e. mirror, will be enough for the power to work.
* The power will work within a 270 meter radius of the user.
* The user may give as many commands as he or she wishes while direct eye contact is maintained.
* The user must give the commands verbally. However, the power of Geass will control the victim for the duration of what the user's command indicates, so the victim may still act under the power's influence even when direct eye contact is broken.
* The user may use the power on as many people as he or she wishes simultaneously, provided that they meet the above rules first.
* The user can also use the power on him or herself, as long as the above limitations and conditions are met.
* The victim will not retain any memories from the time during which he or she is under the influence of the power of Geass due to the rewiring and damage to the brain.
* The power will not work on C.C.
EDIT; 9-8-08. Copied from Wikipedia.
Lelouch's Geass, given to him by C.C., grants him "the power of absolute obedience," allowing him to plant commands within a person's mind upon eye contact which they will obey without question. Activation of his Geass is visually represented by the manifestation of a Geass sigil in his left eye. Commands dictated in this state are written into the minds of the designated targets once the sigil projects from his eye to theirs.
Lelouch could initially toggle Geass activation at will, but subsequently loses this control to an affliction known as Geass runaway (ギアス暴走, giasu bousou?). In the second season, C.C. gives Lelouch a contact lens that blocks his Geass, but indicates that his Geass will eventually grow powerful enough to render it ineffective. Lelouch's Geass evolves to cover his other eye in episode 21, but he receives a second contact to compensate.
Of all the Geass abilities in the series, Lelouch's has been explored the most, and also seems to have the most restrictions and side effects of any Geass introduced.
* Commands must be issued verbally.
* The victim must make direct eye contact for commands to be issued. The maximum effective distance is 270 meters. Line of sight via a reflective surface is sufficient for the effect to occur.
* Commands may be issued only once to any given individual, but any number of commands may be issued at initial application so long as eye contact is unbroken. Since Jeremiah's Geass Canceler negates all Geass effects, those exposed to it may be commanded once again.
* The victim will not perform any action disallowed by their physical or mental capabilities, though they will try to carry out the command to the best of their ability. For example, a victim will not be able to correctly answer a question they don't know the answer to, but will direct the user to someone who can if they are able. However, commands that affect the mind or consciousness, such as telling the victim to forget something, will take effect despite the fact that the victim could not normally force themselves to do so.
* An action will be carried out for as long as dictated in its command, or upon the indicated conditional circumstance. Eye contact does not need to be maintained for command execution to occur. No upper limit has been established, but the conditional command Lelouch gives Suzaku is still in effect more than a year later.
* The victim's memories for the duration of command issue and execution are sealed and cannot be recalled.
* As demonstrated in Princess Euphemia's response to the command that she kill the Japanese, a victim may resist a commanded action to some degree if it is utterly repugnant to them.
So long as the above conditions are met, commands may be issued to anybody besides those who have gained the power of immortality and to any number of individuals at once. Those under the effect of Geass have a red outline on their pupils, presumably a visual cue for the viewer since the characters never point it out.
Since Lelouch is from around... episode twenty of R2, new information needed to be added. Lelouch is an incredibly spoiler-ific character, so I will put his entries under a cut if need be.