This question is not an opportunity for you to vent your frustrations about past co-workers. Questions like this are extremely important in interviews, because they let your interviewer delve deeper into your personality, beyond mere traits and into your own, applicable experiences, which speak loads more than what you think you know about yourself.
For this particular question, focus not on the ways your co-worker was being lazy, but the ways in which you intervened and what the outcome of your actions were – hopefully positive, otherwise you should opt for another story.
For example: One day, I noticed my co-worker John was avoiding a task that he had specifically been assigned that morning by our boss. Our boss said she wanted it done by 3 o’clock, and that time was fast approaching. Worried about what the consequences of missing this assignment would be, I asked John why he had not started on it yet, and if there was anything I could do to help. It turns out, nobody had explained to him how to get started on this assignment, and which piece of software to use for it, and he was too embarrassed to ask. I was able to help him out, boost his confidence in the process, and make a new friend at work!