This paper presents the author's derivations system, an interactive performance system for solo improvising instrumentalist. The system makes use of a combination of real-time audio analysis, live sampling and spectral re-synthesis to build a vocabulary of possible performative responses to live instrumental input throughout an improvisatory performance. A form of timbral matching is employed to form a link between the live performer and an expanding database of musical materials. In addition, the system takes into account the unique nature of the rehearsal/practice space in musical performance through the implementation of performer-configurable cumulative rehearsal databases into the final design. This paper discusses the system in detail with reference to related work in the field, making specific reference to the system's interactive potential both inside and outside of a real-time performance context.