1. M-s h r something
2. Modify buffer.
3. Revert buffer.
4. M-: hi-lock-interactive-patterns
   Verify that variable is non-nil.

5. M-x hi-lock-mode.  (For now do it by hand.)
6. See that highlighting kicks in.

My expectation was that once `hi-lock-mode' is turned on
`font-lock-mode-hook' will be *automatically* called.  

Is it the minor mode's responsibility to explicitly call it's
fontification handler?

>
>>    Problem area: `font-lock-mode-hook' is not documented and I really
>>    don't when it is getting called.
>
> font-lock-mode is a normal minor-mode, so font-lock-mode-hook is called
> at the end of running font-lock-mode (both to enable and to disable it).
>
>
>         Stefan