But that requires one side speaking. If they refuse to speak, then we can't know their thoughts. And people can also tell lies with their voices. Now if we could somehow interface directly with the brain...
Voice is like an API the person provides to others. Like twitter's API for users access data. But twitter can shut down the API or manipulate their API to show us what they want us to see. But imagine if you had direct link to their database?
Think about a person--someone you may already know well--that actually believes his own bullshit. Such a person armed with a brain-to-brain interface is an existential threat to objective reality.
Kind of like we have different voices, accents, speech patterns, languages and manners of speech, I can't imagine our thoughts, even our most accurate representations of the simplest picture (eg "think of a white square"), aren't similarly different.
A friend of his tried and couldn't do it, but was able to hold a conversation while timing a minute, which Feynman couldn't do.
They realised that while Feynman was counting the minute, he was counting verbally (but silently), while the friend was visualising a counter - or Feynman was using the speech centre, so could do a visual task at the same time, while the friend was using the visual centre, so could do a verbal task.
Something as simple as counting in their head was done completely differently.
This is why learning and maturation take so much damned time.