If you would like to know who I see when I'm writing these characters, here you go...
Olivia (on a really good day when she's going somewhere. I imagine Olivia's eyes are green.) This is American actress Lucille Ball. (1911 -1989)
James ( as he was when they were having their affair) This is English actor James D'Arcy.
Agatha (when she's being nice) This is American actress Agnes Moorehead. (1900-1974)
Brooks (all the time, really) This is English actor Martin Freeman.
Sykes in full battle gear (minus the bagpipe)
Pritchard as he looks when he's working in the Hutch (but probably with the coat unbuttoned)
McCrory (when he was in active service)
Rhys-Jones when he was still on the legs God gave him