Playing the Other
When you play the Other, begin by drawing a card from the needs deck. This will give you a broad category for the need driving the Other. You might have only some basic facts about the person you play, but use that to narrow down that general category to something more specific. For example, if the Other takes the form of a bear and you draw diamonds, you might conclude that the bear wants food. Make sure you have the Other's need clear in your mind, and then introduce her as the main character would see her (or experience her, in the case of some less-corporeal persons) as she acts on that need.
Whenever you play a character other than your own, you have the goal of making her seem real. Take everything we know about her (which sometimes might not give you very much at all) and portray her as a real person. However, you cannot make decisions for her. The skill in playing the Other or a secondary character that doesn't belong to you lies in portraying a character while stopping short of making any decisions for her. You can tell us about things she says and does, so long as they follow from what we already know and do not constitute a substantive decision. If you reach the point where she must make a decision one way or another, someone needs to ask a question or make an appeal.
The Other's need counts as something you know about her. You chose the specifics of it, but the cards pointed you in one direction or another. Even if the Other wants to keep her need a secret for some reason, a perceptive person might see through the charade.
If other players spend awareness to ask questions, you must answer truthfully. Not all questions will speak directly to the need you chose, but if it does, answer the question truthfully. Don't worry about creating a puzzle or making sure that your friends can't guess too easily which card you drew. Even when you try to communicate it as clearly as possible, you'll find it sometimes difficult to do so. You don't need to try to make it harder.
Encountering Other Main Characters
You can encounter other players' characters as the Other. None of the above changes, but you do have a few more restrictions. Your card and hers must both appear at the same place, per the normal rules. She must play her character as the Other. No one else gets to take over her character because you choose to encounter her.
When someone chooses to encounter your character, you still draw a card from the needs deck. This represents your present need. We might have just seen you in an encounter focused on solving a mystery, learning about a place, or talking to someone, but needs can shift quickly. If you draw clubs, perhaps the spiritual ramifications of recent events suddenly weighs on you. If you draw hearts, perhaps you have a sudden rush of emotion that you find difficult to contain. If you draw diamonds, perhaps you suddenly become hungry or thirsty or tired. If you draw spades, perhaps your efforts to put it all together and understand what has happened leaves you feeling lost and confused. You can try to connect the card you draw to the existing narrative, but if you find it difficult, remember that things can change quickly. Don't feel like you can't introduce a sudden shift.
Unlike playing other characters, you can make decisions for your own character, even as the Other.