The easiest way to teach new players how to roleplay is simply to throw them in the deep end. Don't bother going through a bunch of rules or details about character creation, or what dice to roll in what situations. Just explain that everyone is playing the role of a character and start playing. When they need to roll some dice, hand them the right dice and tell them what they need to roll. That's all the rules you really need to know when you start.

You can run games with little kids this way. The game Toon is especially good for this. You don't need to tell them anything about how the game works... Just say, "You're a rabbit, you're a cat, and you're a tweety bird," and let them go.

(A lot of the "roleplaying advice" given in these annotations is very tongue-in-cheek, but this one is actually serious. Yes, we need a special note to indicate when we're not being silly...)

Transcript

R2-D2: So Jim, do we meet your new character here too?
Captain Antilles: {dead} No, I'm not done with the background yet.
GM: And I need to find a way to work him into the adventure.
Captain Antilles: {dead} It's really detailed.
GM: Which is why I need the time. Anyway, Adam, your father is looking over the droids for sale when you hear your mother, Beru, call from the homestead.
Beru Lars: Adam!!
Adam: Uh... Whoa.
Beru Lars: Adam, tell Owen that if he gets a translator to be sure it speaks Bocce.
Adam: What the—
Owen Lars: You, I suppose you're programmed for etiquette and protocol.
C-3PO: Yes. And I can teach him poise and dancing as well.
Adam: Hang on. Who... What?
C-3PO: I'm going to have to charge you double.