Is there a life hack to get a comfortable seat in a flight for free?

You cannot get a seat freely from business class section, even if that section is empty. But if you are traveling, go for the airline check-in at a very early time, request for a seat next from the row next to to emergency exit, which will have extra leg room and will be more comfortable than the usual seats. Depending on the availability, you can get a seat from this row without an extra charge.