Who should I prepare more for an interview, the HR director or the hiring manager?

HR does know liars and BS artists. Does know when the dots don’t connect when you relate stories about past jobs.

Let’s call them HR and HM.

If you’re interviewing with HM, who may later be your boss, HM is going to know exactly what work you’ll be doing day to day. Present with deep knowledge and immersion in your work.

HR, on the other hand, knows what HM wrote about the job and maybe no more. HR does know liars and BS artists. Does know when the dots don’t connect when you relate stories about past jobs. Does know whether the job calls for this personality or that. If you’re late or otherwise unprofessional, that’ll get noticed, too. Present with the simplest dignity. Same for any upper management you meet.