This is the post to check if you want something cleared up about Lelouch's Geass, or want to come plot with it!
First off, the rules about the Geass. They were kind of glossed over in his app, so here they are:
1. Lelouch must be making eye contact for it to work. It works if he is looking at you through a mirror or transparent material as well, though that fails at greater distances. Its maximum range is 270 meters.
2. The Geass has the power to make anyone obey one absolute command. Once this power is used on your character, your character cannot fight it, except when given an order that goes against the very fiber of your character's being. This is very, very rare, only half-effectual, and not likely to happen at all. Even if a character is able to fight initally, they will eventually submit. Your character will not remember anything from a few seconds before the command is issued to a few seconds after it is carried out. They will not remember what it was. They will not remember what they did. They will not remember Lelouch, unless they had seen him a while before the Geass was used.
3. A command must be issued verbally. Lelouch does not have to utter a direct command; he can phrase it as a suggestion or request, so long as eye contact is maintained, the Geass is activated, and his intention makes the command clear. Your character's personality, memory, and free will will be utterly crushed. They will not be able to "interpret" a command's wording in a manner different from its intention.
4. Your character cannot be made to do something that is physically or mentally impossible for them. For example, if your character is made to answer questions truthfully, and Lelouch asks a question that your character doesn't know the answer to, they will not be able to answer. If they know who can answer, they will be able to give him that name. You get the idea. As for physically impossible, your character may try to complete the task to the best of his/her ability. (Don't worry about that one; Lelouch is pretty good about these things. <.<;;)
5. The Geass will only work ONCE PER CHARACTER. During the initial Geass contact, Lelouch can issue as many subsequent commands as he needs to, but once eye contact is broken, it's done. He cannot countermand an order, as that would involve using the Geass a second time. Again, if Lelouch has already used his Geass once on your character, it will never work on your character again.
6. Lelouch's Geass will grow stronger with every use. This really has no bearings on your characters, honestly, it's just here because it may be important, especially since if Lelouch overuses his Geass, eventually he will lose the ability to turn it off. EDIT: This is now the case. Lucky for everyone involved, Lelouch has a special contact lens that suppresses it, so there shouldn't be any "lol woops massacre" incidents again. Shouldn't. :|
Also, for reference, when Lelouch activates his Geass, it looks like this:
Note that you can't actually see it "in-universe"--this is just how his power is visually represented in the show.
And now, plot ideas! If you want your character to be Geassed by Lelouch, as part of a plot (or as a result of circumstances, blah blah blah), just post right here and we'll discuss it! Note, of course, that once your character has been Geassed by Lelouch, they can never be Geassed again.