Matthew Marsh played DCI Mike Hedges for the first four series before deciding to quit the role, and was replaced by Mark Lewis Jones as DCI Doug James for the final two series.

Amanda Burton (as Commander Claire Blake), Anthony Valentine and Paul Brightwell were the only original cast members remaining by series 4, after several cast changes across the run.

The role of DAC Edward Sumpter was originally played by Christopher Robbie in series 1 but was recast with Anthony Valentine from series 2 onward.

The role of DCS Les Branton was played by three different actors across the series: Nicholas Jones in series 1, Pip Torrens in series 2, and Nicholas Jones again upon the character's return in series 4.

The very first series 1 story had no official title during its original broadcast in February 2003; it was only officially named for its DVD release alongside series 2 in 2005.

Series 3 and series 5 were never released on DVD in the United Kingdom, with series 3 falling in a gap between two different DVD distributors, Contender and Acorn Media UK, which acquired the rights in 2007.

Midway through the show's run, Ofcom and ITV received numerous viewer complaints about Commander Claire Blake's willingness to sleep with suspects to obtain evidence, notably in her relationship with murderer James Lampton, played by Hugh Bonneville; many viewers found this plot device unbelievable.

Later series of the show changed format, moving from single feature-length two-part stories to multiple separate stories per series, such as three distinct two-part stories in series 4.