certain dialogue scenes
This was a brilliantly conceived film and I could find no fault.
What I found particularly interesting were the dialogue scenes between Phillip and Kari when he was attempting to resume/relive their relationship. There were a number of instances where they were speaking, but by means of a voiceover (in the park in Oslo, in the cafe and hotel room in Paris); the visuals showed them not speaking, simply looking.
The effect it had was that the two timelines were becoming intermingled past the point of distinction -- both for me, the viewer, and presumably also for Phillip. The forgotten, distant past became simultaneous with the present.
This is the first film I've seen by Joachim Trier (or Eskil Vogt, for that matter), but Reprise left a lasting impression.