  Who sets the policy agenda? In this paper, we explore the roles of policy
actors in agenda setting by studying their relative influence in policy-related
discussions. Our approach builds on ``nodality'' \textemdash a concept in
political science that determines the capacity of an actor to share information
and to be at the centre of information networks. We propose a novel methodology
that quantifies the nodality of all individual actors in any conversation by
analysing a comprehensive set of their centrality measures in the related
information network. We combine this with the analysis of the activity
time-series, of the related conversation (or topic), to demonstrate how
nodality scores relate to the capacity to drive topic-related activity. Here we
analyse policy-related discussions on X (previously Twitter) and quantify the
nodality of two sets of actors in the UK political system \textemdash Members
of Parliament (MPs) and accredited journalists - on four policy topics: The
Russia-Ukraine War, the Cost-of-Living Crisis, Brexit and COVID-19. Our results
show that the capacity to influence the activity related to a topic is
significantly and positively associated with nodality. In particular, we
identify two dimensions of nodality that drive the capacity to influence
topic-related activity. The first is ``active nodality", which reflects the
level of topic-related engagement an individual actor has on the platform. The
second dimension is ``inherent nodality" which is entirely independent of the
platform and reflects the actor's institutional position (such as an MP in a
front-bench role, or a journalist's position at a prominent media outlet).
