What I find always extremely interesting when watching British movies is the way they deal with class politics. The IPCRESS File is rather subtle about it, so subtle that I'm not sure how much someone not looking for the intricacies of class relations would get out of them here. Michael Caine's Harry Palmer is of course part of the working class, but of a very specific kind of working class person who is more educated, competent and intelligent than most of the people he's working for. Palmer's sarcasm (or "insubordinance") shows very clearly how conscious he himself is of the fact that it's his upbringing that will always hinder him being more than a footsoldier; he just doesn't seem willing to be all that bitter about it, even when his superior Ross (Guy Doleman) uses him as tool.
Ross, on the other hand, is interesting in that he's more of a professional than of an upper class twat, although his scenes with Dalby (Nigel Green) - as upper class as they come - show where he initially comes from and how good he speaks the language of that place. He's an upper class man in transition to competence yet still as morally bankrupt as his class tradition demands, using his own private working stiff to do his dirty work for him.
Also of interest (and just pretty damn cool) is the film's aesthetic, at once as far away from the also Saltzman-produced Bond films as possible and very much in the same spirit of style. Where the Bond films go for the exotic and the colorful, The IPCRESS File uses the mundane and the brownish grey, yet both series are equally stylized - the Palmer films just make an art out of pretending to be artless.