Source: "Monolingual Fieldwork" Demonstration - Daniel Everett (YouTube)
Summary from UWElingo blog post:
In this video, Dan Everett is doing a ‘monolingual fieldwork presentation’, something quite impressive: he works with a speaker of a language that he doesn’t know (and which was chosen in secret by others, so he couldn’t prepare for this). Dan is showing that without having a language in common, it is still possible to do Linguistic Fieldwork. In this case, Dan is speaking Pirahã, to ensure that the lady he is working with won’t get any additional clues through his language use.
This is a great example of how you can do monolingual fieldwork – in particular how to start monolingual fieldwork.
After the presentation, there are numerous questions from the audience (again: really helpful in showing how this works). Also, it is revealed that the language is [REDACTED]!
He may be speaking Pirahã, but he's taking notes in English on the chalkboard, and the woman he's talking to appears to be reading his notes and adjusting what she's saying as a result.
There was a very similar comment in the reddit post, to which I replied: