Anyone can join in a public scene on the community journal. If you want a more private interaction between your character and another, you can post on your character's journal to start a scene.
You can also use your character's journal to post diary entries, letters, etc.
As of 12 Oct 2003, canon characters who need players include: Jean Valjean, Javert, the Thenardiers, Azelma, Gavroche, Combeferre, Jean Prouvaire, Joly, Laigle (Bossuet), Bahorel, Feuilly, Musichetta, M. Mabeuf.
No powerplaying. This is also known as powerposing or godmoding. You are only allowed to decide what your character does; you can't dictate what other people's characters will do. If you say Montparnasse pushes Azelma down the stairs, without taking into account that Azelma might get out of the way in time, you're powerposing.
No abusing the "no powerplaying" rule! You can't play along up to the last minute and then back out of an interaction -- so if Azelma agrees to fall down the stairs, she can't then say she doesn't get hurt at all.
No supernatural/magical stuff. While that can be fun, it's not for this particular game. Vampires are out; wizards are out; telepaths and sailor scouts are out.
Be civil to other players. If you have a disagreement with another player, work it out in private; don't argue on the community. unlimitedrpg.com has a handy private messaging system. If you can't resolve your problem, the moderators will resolve it for you.
Don't spam! Use the "comments" feature to join in a scene instead of starting new posts every few minutes, and don't post off-topic messages in the community.
That's all!