This show is a reunion for Karen Gillan, Alex Kingston and writer, Steven Moffat. The trio previously worked together on Doctor Who when Moffat was the showrunner.
Hugh Bonneville and Karen Gillan were both in episodes of Doctor Who together. Episodes: "The Curse of the Black Spot" and "A Good Man Goes to War"
Alex Kingston played River Song in Doctor Who, she played Karen Gillian's character's daughter in the series.