If you’re writing a series, be sure to remind your readers what your characters look like. This also pertains to any key plot elements that happened in the previous episode(s). To you as the author, it’s all one story. To the reader, however, weeks, months, or even years may have passed since they read the first book. Thus, they may have forgotten numerous details, like what the characters look like, or other important details.
Furthermore, it’s also possible that someone will start reading in the middle of your series. These new fans definitely need this information! If they’re lost, then what? Best case, they’ll buy the previous book(s) and read them in order. Worst case, they put it aside and you’ve lost a potential reader.
It’s best if each book in a series has it’s own independent plot, even if it’s part of a larger picture. Flashbacks or character dialog covering previous events are ways to sneak in information they missed. Descriptions of their appearance and perhaps the setting itself should include reminders at least. Your readers will thank you for it.