In the Acts of the Apostles, a number of verses portray the Holy Spirit as a character who directly acts and intervenes at several points of the narrative. The Spirit speaks (e.g. Acts 8:29, 10:19), descends and inspires glossolalia (e.g. 2:4, 19:6), appoints to particular tasks (13:2, 20:28), snatches Philip away (8:39), etc. This study examines these instances (20 verses in total) from a narrative-critical perspective in order to answer the question on how these actions indirectly characterize the Holy Spirit. For this purpose, an exegesis of each verse, particularly the verbs denoting the actions of the Spirit, is conducted. The implications of these actions for the narrative – what we call their narrative functions – are investigated. The thesis demonstrates how significantly these actions fashion and unfold the plot of Acts, and argues that any attempt at structuring the narrative would have to account for this fact. It also attempts to evaluate four of the most prevalent structures which have been proposed by scholars for the Book of Acts in terms of the place and function of the direct actions of the Holy Spirit in the narrative of Acts. It is argued that this plot-determining role of the Holy Spirit is best accounted for in C.H. Turner’s six panels. These need to be considered in determining the most appropriate structure of Acts. In conclusion, the Holy Spirit, as characterized indirectly through the actions in the Acts of the Apostles, is best described as the Spirit of witness. The Holy Spirit as the supreme witness to the risen Jesus functions as the director of the mission, and as the source of power behind the growth of the Christian community and the expansion of the witness to the ends of the earth.