@Magic-MRX said:
Films or series an actor is 'known for'
I want to pick movies or series that a specific actor is 'Known For', how do I do that?

When you make the API Request using search with a person's name
in the response are the first 3 movies of that know_for category that are on the person's page.

https://api.themoviedb.org/3/search/person?api_key=THE_KEY&language=en-US&query=Pedro+Pascal

The selection of films in know_for I believe is based on popularity, votes and some other criteria of the films.
Then it is possible to make a Discover API Request, sorting by popularity
and using the person ID in the with_cast, with_crew or with_people filters.
The answer is not exactly the same but it is similar to know_for.
Example with actor Pedro Pascal, ID 1253360, using with_cast.

https://api.themoviedb.org/3/discover/movie?api_key=THE_KEY&language=en-US&sort_by=popularity.desc&page=1&with_cast=1253360

In addition there are plans to add this feature.
It's already on Trello's To-Do list.
There is no implementation deadline.
You can vote. More votes, faster. :-)
See here:
https://trello.com/c/eM2R9Dxu/125-add-a-known-for-field-method