– “that” is for things; “who” is for people. I’m probably not the only one who winced a bit the first 100 times I heard Katy Perry’s great song, “The One That Got Away.” I even heard the radio announcer saying, “It should be ‘The One WHO Got Away,’ of course!”
(This one’s directed at North Americans, as Brits use “which” where we use “that,” so they have their own rules.)
the doctor, but Doctor Wilson; the president, but President Obama; the general, but General Eisenhower; the judge, but Judge Judy; the sergeant, but Sergeant Wilson; the prince, but Prince Charles; the police department, but the Chicago Police Department; the library, but New York Public Library, the hospital, but Toronto General Hospital.
See also my blog posts, “Style Blunders in Fiction” and “Hyphens, Dashes,Ellipses.”