returning regulars 是什麼意思

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The context here seems to be one of the latest Marvel movies - these movies are known for having the same people eg Robert Downey Jr, Chris Hemsworth, appear in many of them. People like this come back (“return”) on a regular basis to appear in these movies, so may be referred to as returning regulars.

The reason Cumberbatch does not refer to anyone by name is that it could be considered a spoiler (he is very likely contractually obligated not to give anything away without clearance). So he just says that the actors who come back (again) in this movie are great.