I understand that one is supposed to follow along with the communal Torah reading in Hebrew. But is this actually a requirement, as opposed to just a good thing to do? Another way of asking the same question: is it a sin to study the commentaries on the parshah during the reading instead of following along with each word?
I have the same question regarding the megillah on Purim. If I'm there at the megillah reading and studying the commentaries to the megillah while listening, have I fulfilled the mitzvah of listening to the megillah reading? In such a case, I would have been hearing the words as they were recited, even though I was concentrating on the commentary I was reading.
I'm not assuming the two question must have the same answer. I'm just asking these questions together because they're parallel questions.