The 1940 Census lists answers to several new questions never asked before including where they lived in 1935 and what was their income for the previous year.
Be sure to look at several families before and after the family you are researching. These people are most likely the friends or family of your ancestor. Many lived in the same community very near each other.
In addition to the census population count, there are a number of special censuses: Slave, Industry & Manufacturing, Agriculture, Mortality, Social Statistics, Union Veteran and Widow, Defective, Dependent and Delinquent.
Both Civil Registration and Census Returns run out when you get back to about 1840, and rarely provide information relevant before 1800. At this stage you need to turn to Parish Records: These date back to the mid 1500's. Cromwell, at the Court of Henry VIII, ordered that every wedding, baptism and burial should be recorded.
Consider using video tape for your interviews. Those being taped must approve this type of interview.