[@AccursedBiscuit]
Good Question! Basically you can speak to them, observe them, or take an action with a certain intention. 
You can never make another character talk or act. you can only do that to with your own character. 

Basically this means you can write:
Bob (your character) said "hi" to Tom (other persons character)
Bob stretched out his hand to shake Tom's hand. (with the intention of shaking toms hand, but not actually describing that action because that would need Tom's player's permission)
Bob saw that Tom was very fat (if that's in the characters description)

But you [u]can't[/u] write:
Tom (not your character) said: 'Hey bob, I think you're so cool. Here is all my money.'
Bob punched Tom in the face. 
Bob saw that Tom was turning purple (when the player of Tom didn't state that)

While I'm not going to write out the entire etiquette, you could google on 'godmodding' for all things that you shouldn't do.
We're also on the skype btw. Which is probably a more easy way to talk.

[edit: And this: http://www.roleplayerguild.com/posts/3337607 exists, but it's not really about how to RP with other characters, but more a general RP etiquette]