Which just goes to show that there's often a lot going on beneath the surface of even the quietest people. The strategies below should help you draw out your co-worker. It's not enough to listen. You've got to show her that you're listening. Maintain eye contact, postpone interruptions and ask specific questions based on what she's said.
People refrain from interacting for many reasons.
Maybe she's afraid to speak up around the boss. Maybe she's nervous in a group. Maybe she's afraid to speak for fear of being criticized. If she warms up to you all individually, she may become more outgoing in the group. When I first began making speeches I couldn't understand why audiences didn't laugh at my jokes.