so who carries out the interview...I would suggest it is a detective who reports to the lead...they have a discussion and go from there. It is not the chain of people who do not talk to each other as gunit is suggesting
The SIO runs the investigation. They decide everything and their team follows their directions. The SIO has lots of advice to call on if required and they report upwards. Some interesting info here, particularly about the weight given to different kinds of evidence;
A clear understanding of the reliability of the material will enable investigators to determine the weight they should give to it in the evaluation. The following may assist investigators to determine the appropriate weight a piece of material should be given:
• Material that can be corroborated by an independent source of material will have high reliability;
• Material that can only be corroborated by a person such as a spouse or other relative will have less reliability;
• Material that cannot be corroborated and conflicts with other material gathered in the investigation, will have less reliability;
• Material indicating other factors which may cast doubt on reliability of the material.
http://library.college.police.uk/docs/APPref/core-investigative-doctrine.pdf