A formative influence on Casserley's work came in 1963, when Karlheinz Stockhausen, Christoph Caskel and David Tudor visited Chicago. One of the works they played was "Kontakte", and hearing this work for the first time impressed on him the potential of the electronic medium. His first experience of computers came in 1966, his final year at Chicago Musical College, where he took a course in the use of computers for musical analysis. His project for the course was a motivic analysis of Varése's "Density 21.5".
In 1967, while a post-graduate student at the Royal College of Music, he joined a new course in electronic music founded by Tristram Cary. The RCM studio had close links through Cary with Peter Zinoviev's pioneering computer studio in Putney. In 1969 the material for the tape part of "Solos, Commentaries and Integrations" was created on Zinoviev's system, then further processed and montaged at the RCM studio.
From the first Casserley's major interest was in utilising electronics in live performance, and that has remained his prime concern. Throughout the 70s he worked with electronic performance in a range of contexts, collaborating with musicians, poets and visual artists as well as performing his own compositions.
In addition, he worked on the performance of many compositions involving live electronics. At the Royal College of Music he developed a close association with the Twentieth Century Ensemble, founded at the same time as the studio by Edwin Roxburgh. Casserley was in charge of the sound for many important Ensemble events, including the UK premieres of Stockhausen's "Mixtur" and "Trans", and performances of many other works by Berio, Halffter, Lambert, Gehlhaar and others. He was also instrumental in creating the electronic portions of scores by other composers, notably Edwin Roxburgh's "Saturn" and John Lambert's "Sea Change" cycle.
In the 70s he worked on a number of electronic design projects, including a design for a low-cost, microprocessor-based polyphonic synthesiser. By 1980 he was working on ideas for a real time digital signal processor. This work continued throughout the 80s, during which he produced several interesting prototypes, but was unable to find the resources to bring them into useable form. By the early 90s he was determined to stop counting nanoseconds and get back to making music. The appearance of the IRCAM Signal Processing Workstation in 1992 gave him the chance to eat his cake and have it too!
Since that time he has used the ISPW to re-create earlier live work by himself and others, to create new work, including his own compositions, collaborative work and realisations for other composers. Most particularly he has used the potential of the ISPW to develop new approaches to performance, which are finding a particular application in improvised music.
Since 1998 he has developed his computer instruments on MacIntosh computers using Cycling74's Max/msp software. This has included instruments for performance and interactive installations. In particular, he has developed a series of Signal Processing Instruments for improvised music.