Observations in Chapter 1 about the Cabinet system in general apply also to the Prime Minister. His functions and powers are a matter of convention, not statute, and consequently are exceptionally flexible: the parameters of his role at any given moment depend on the circumstances of the moment and on the personalities and aptitudes of leading actors within the system. While any actor is constrained by the institutional restraints imposed by the role he fills, the constraints on the Prime Minister are looser than on most British politicians in elected office. It is only possible to set out the practices that seem to apply in most circumstances, and describe the repertoire of roles that a premier may undertake. Any explanation must be applicable to a variety of personalities from the relaxed post-war Churchill to the interventionist Mrs Thatcher, and to a spectrum of circumstances from the placid 1950s to the fraught 1970s.
Strangely, the Prime Minister arriving at No. 10 from a Whitehall department can find himself comparatively under-employed. A department provides a ready-made workload. In contrast, the Prime Minister has few duties that he must carry out. He is responsible for everything, yet obliged to do almost nothing. Callaghan observed in retrospect:
To a large extent the Prime Minister makes his own pace. It is the Prime Minister himself who takes the initiatives, who pokes about where he chooses and creates his own waves. Ideally he should keep enough time to stand back a little from the Cabinet’s day to day work, to keep in touch with Parliament and outside opinion, and to view the scene as a whole, knowing full well that periods of crisis will occur when this will be impossible.