Under a time charter, who would pay the crew's overtime when work had to be done preparing the ship for a particular cargo, e. g. washing down holds?

There may be an agreement in the charter party for an extra payment to be made by charterers to owners each time the crew sweep and/or wash down the holds. The same would probably apply for lashing a ro-ro or container cargo, etc. If charterers require ship's crew to do extra work for their purposes, they probably pay.