"형사가 우체부를 면접할 것입니다."
Translation:The detective will interview the postal worker.
They both mean will do. The 하겠습니다 structure does have the nuance of a more concrete intent to do something in future, but it is also a lot more formal and is used mainly with the 니다 form.
할 것입니다 means will do also but is less formal and carries a little less concrete intent to do something in future - however that doesn't mean it carries the connotation of probably or maybe doing something.