I have a question about the difference between the verb "cover" and phrase "fill in" in couple of work-related contexts. The common feature of the two contexts is a store with two employees, John and Jane. On some days of the week, only John is on duty. On some other days of the week, only Jane is on duty. For all the other days of the week. Both John and Jane are on duty.
In one context, on Mondays Jane is normally on duty and John is normally off. Suppose for one particular Monday, Jane could not make it to work, so John had to come in:
1a. John covered for Jane on Monday.
1b. John filled in for Jane on Monday.
In another context, on Tuesdays both Jane and John are normally on duty. Suppose for one particular Tuesday, Jane could not make it to work, so John had to shoulder her workload for the day, in addition to his own workload:
2a. John covered for Jane on Tuesday.
2b. John filled in for Jane on Tuesday.
Is one of "cover" or "fill in" more appropriate in sentence pairs 1a-1b and 2a-2b?