“Too Close” star Emily Watson has a personal relationship to the source material for the three-part psychological thriller, premiering Thursday (May 20) on AMC+.It’s based on the titular 2018 novel by Natalie Daniels, the pen name used by Clara Salaman — one of Watson’s oldest friends.“I’ve known her for pretty much 50 years. We were at school together,” the British-born Watson, 54, told The Post.