When a book series contains the same characters, to what extent is it necessary to re-introduce the main characters to the reader (in case, for example, they haven't read the previous books)?
I've read the entire spectrum of this. From awful, tedious cases where each character is re-introduced in painful detail for the duration of the first chapter, to very little or no introduction at all where I'm left wondering 'who was that again?' because I'd read the previous book in the series 6 months ago. The best ones are in-between, where there is some introduction but it just doesn't feel like it. It's a natural progression of the story.
Are introductions like this always necessary? How do find the right balance for my story?