Sharon Tate Moody compares the interviewing techniques of two TV detectives:
The first is Sgt. Joe Friday. The stone-faced lead character of the "Dragnet" series never veered from his approach. "All we want are the facts, ma'am," he would say, not cracking a grin or blinking an eye.
Then there was Columbo. He was a fumbler interested in the quirkiest of things about unsuspecting persons of interest, sidestepping his way to what he really was after. I also think he was a closet genealogist. He was forever talking about his cousins and revealing things about his family - things such as his father having been a tail gunner on a beer truck during Prohibition and his grandfather being 40 years old when he began wearing dentures.
Columbo's approach might work well for you when interviewing reluctant relatives. [Link]