When you want to get a better understanding of your user's behaviour, and can observe them in their normal environment.

User's behaviour is often influenced by their environment. Watching users in context can often yield insights that would not have shown up in any other way. This one is harder to do remotely but can still be really productive. You might try things like asking them to take you on a tour before starting.

1. Gain not just permission but acceptance of your presence. People must be comfortable or they will act differently.
2. Look closely at the physical environment. Are there any clues about their routine behaviours? (Post-it note reminders? Calendars on the wall?)
3. Blend into the background and try to be unobtrusive.
4. You can ask questions at the end.
5. Take notes on everything: behaviour, cues, distractions, emotional responses (including non-verbal), other interactions etc. This is harder to do remotely over video - a recording might help you to catch things, but make sure to get their permission!
6. Look from various vantage points.