Before interviewing relatives who were alive at the time of the 1940 census, try locating them in that record. Note the names of neighbors and ask your relative about these individuals. Giving your interviewee specific names may help to jog memories and get them to recall events they might not otherwise have thought about.
This is helpful even if the person was not alive in 1940. Neighbors might have been neighbors for decades and even if the person did not know the former neighbors personally they might remember hearing their name mentioned.
Anything that might help jog a memory is good.