Step into the light where it feels the hottest. Step into the light where it feels the hottest, then take a half step back. You can refuse to do the scene partially nude. Such a request made during the filming of a production is unethical. If nudity was required or contemplated, you should have been informed before you chose to audition.
Rehearsals generally occur first thing in the morning before the day's shoot begins.
A minimum of 1 hour daily rehearsal time is required by SAG rule 37B- a 12. Movies aren't plays. Most movies don't have rehearsals. Heck, you rarely ever meet the entire cast. Any rehearsals you do are on your own and off-set.