When Philip Ashley suggests a picnic to his cousin Rachel, she responds, “If we are to picnic by the sea or sail a boat,” she told me, “I will not come with you. It is too early in the year to sit upon a beach, and as for climbing in a boat, I know even less about that than I do about a horse.” The picnic is kaput. 
But dissatisfied with Du Maurier’s decision to skip the picnic in the novel, My Cousin Rachel, Michell was determined to have one in his film. Influenced by Henry Koster’s beach episode in his 1952 film that is not a picnic, Michell invented one. Like Koster’s, the Michell beach episode is a metaphor for Philip’s “rocky” relationship with Rachel. At Michell’s picnic, Philip confronts Rachel with a letter written by her deceased husband Ambrose, indicating that she is murdering him. Philip cannot believe this damning evidence. Bewitched by Rachel and not accepting that the note is truthful, he burns the message in the picnic fire. The note is truthful.
Featured Image: Rachel Weisz as Rachel Ashley and Sam Clafin as Philip Ashley
*Michell also has picnic episodes in his films Enduring Love and Hyde Park on the Hudson.
See Roger Michell. My Cousin Rachel (2017). The screenplay by Roger Michell is based on Daphne du Maurier’s novel; Daphne du Maurier. My Cousin Rachel. London: Victor Gollancz Ltd, 1952; Henry Koster. My Cousin Rachel (1952). The screenplay by Nunnally Johnson is based on Daphne du Maurier’s novel (1951). Du Maurier’s text is available online at https://archive.org/stream/in.ernet.dli.2015.202230/2015.202230.My-Cousin#page/n229/mode/2up/search/picnic