Actually, John Adams had a house in town as well as the farm in Braintree. His law practice had become so active by 1768 that he needed an office in Boston. He had moved his family to Boston (the first of several back-and-forths between farm and city) well before the Boston Massacre.
The movie does not show him witnessing the riot--only some of the aftermath. (No spoiler here, I hope, as this takes place in the first few minutes of the first episode.) The show has him at home on that night, but in reality, Adams had been in the company of a "Clubb" of other gentlemen. Here's what John Adams wrote of that night in retrospect:
About nine O Clock We were allarmed with the ringing of Bells, and supposing it to be the Signal of fire, We snatched our Hats and Cloaks, broke up the Clubb, and went out to assist in quenching the fire or aiding our friends who might be in danger. In the Street We were informed that the British Soldiers had fired on the Inhabitants, killed some and wounded others near the Town house. A Croud of People was flowing down the Street, to the Scene of Action.
When We arrived We saw nothing but some field Pieces placed before the south door of the Town house and some Engineers and Grenadiers drawn up to protect them.
So, yeah, there was a little liberty taken with the facts, but not enough to bother me--so far.
I echo everyone else's sentiments about the quality of this programming, and I hope HBO is taking notice. I have done without HBO and Showtime for years. The Sopranos didn't entice me enough to subscribe. Deadwood didn't. Even Six Feet Under wasn't enough.
But when "John Adams" came along, I added HBO to my programming package. Content this intelligent and well produced deserves to be supported. Bravo, Tom Hanks.