Motor functions and cognitive processes are closely associated with each other. This linkage is reflected in motor system state changes both when an action must be prepared or stopped. In the context of single-pulse transcranial magnetic stimulation protocols, both action preparation and action stopping are accompanied by a reduction of corticospinal excitability, referred to as preparatory and response inhibition, respectively. Yet, despite their apparent commonalities, a direct, comprehensive, and systematic comparison of the two phenomena is lacking. To ameliorate such deficit, this review will focus on the role and interpretation of corticospinal excitability changes during action preparation and action stopping aiming to systematically flesh out commonalities as well as differences, detailing how corticospinal excitability is affected by states, traits, and psychopathologies between both processes. Eventually, findings will be compared directly, and open questions will be addressed to aid future research.