I was lucky enough to see the original Steppenwolf production of the play on which this movie was adapted. I have never been so profoundly moved by anything...and I see theatre all the time.
I began crying about fifteen minutes before the end of Act One. I didn't stop through the entire fifteen minute intermission, nor through the entire second act. I finally tapered off during the second intermission, but started back up again halfway through the third act.
Come to think of it, I was still with XABF at the time and we saw it together. I don't know that I was quite processing all the emotion the story (and performances -- Amy Morton, OMG!) stirred up or why, but I do remember him acting rather embarrassed that it hit me so hard. We'd stay together another few months after that -- but maybe if I'd really dug into why I was so affected I would have ended things sooner.