For example the other day I saw an episode where Patrick Stewart said one line. Did they fly him in to record that one line? Or most likely they planned way in advance for that one line and record it earlier?
sharePatrick Stewart generally records his lines at a studio in London but I don't know what the case is for everyone else.
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There are a number of things that they can do. Sometimes they will fly someone in, and have them record multiple lines, episodes; so they get all of the material done in one day.
Or, they will have them record it at a studio more local to them.
Or, they can even record their lines over the phone.