v.1. To enter something by walking: The door was open, so we walked in and took our seats.
2. To enter casually, easily, or unawares: That new student walked in off the street and became the starting quarterback. I don't think you can simply walk in and take over the responsibilities of this new job.
3. walk in on To walk in and see some private situation or someone in a private situation: They were embarrassed when I walked in on them making a mess in the kitchen. The guard walked in on a robbery.