The quickest way to learn spoken Hebrew is to be stuck among Israelis who can't speak any English. I was stuck in a yeshivah for 3 months in a room with two Israeli baal teshuvahs who couldn't speak a word of English. I came out with a real "Israeli" accent and had all the right tshachtshach epithets & swear words down to perfection r"l!
Were any of Chaim's cellmates Israelis, when he was inside?