Complex sentence: Marge, in particular, suspects Ripley's involvement in Dickie's death; when she expresses her suspicions, Ripley prepares to murder her but is interrupted by mutual friend, Peter Smith-Kingsley, who enters the apartment.
Split into simpler sentences: Marge, in particular, suspects Ripley's involvement in Dickie's death; when she expresses her suspicions, Ripley prepares to murder her. He is interrupted when Marge's friend, Peter Smith-Kingsley, enters the apartment.