The pace of this film is very leisured, to begin with, indicating a Hitchcock that is more self-assured than ever. And I suspect the sparse pace in the beginning is calculated to contrast with the tension that is to come. This is an unmistakable invitation to the audience to adjust their mental pace, and participate in the plot development.
A truly remarkable thing is the cinematography: surprisingly modern for its day. The cameraworks is very much like what is usually found in a 1970s film: frequent close-ups; the juxtaposition of Melanie and Annie in different depths, for instance. One also admires Hitch's effort to create sophisticated composite images, in the days without computers---for instance, the blue screen with the (then) new sodium process, and the mat/ silhouette game...... What resourcefulness, and what attention to details! All comes from painstaking planning: Hitchcock likes to make his own continuity sketches; he used to say that he prefer the conceptual sketching over the actual making of the film!
The acting in this film is rather strange. The conversation is cool and poised, very fitting to Tippi Hedren's curious, vacant beauty (she started his career as a fashion model). Every time I watch this film, never mind how many times I have already done so, I am always amazed by what the feminist critic Camille Paglia terms the "mesmerizing narcissism" of Tippi's. By the way: have you noticed that Hitchcock's heroines are always so self-possessed, sexually?