The set-up is that Jackie is giving an interview a week after the assassination of her husband, so we see the events she’s describing in flashback while coming back to the interview itself periodically.
I have no idea if Natalie Portman does an amazing job here -- I know zero about the actual Jacqueline Kennedy. I do know that it’s a very precise
performance and that she seems to be portraying a woman whose every public action was a deliberate performance. So, though it may be accurate, the whole thing also came off quite artificial and very, very cold.