When it gets a bit frisky and somebody screams, a compressor catches it. But you can always hear it working. I watch films and think that worked and that was a disaster. I deliberately haven't used a compressor on any dialogues in four years unless it's something really extreme. He said 'No'.
Compressors are a lazy man's way, a catch-all thing.
Dynamic range is reduced and so are the top and bottom frequencies. I also asked him where he put his dialogue filters. On this last film, I took 20dB at 1kHz out of the Foleys because I've got this theory, probably completely flawed, that Foley people in this country whack their foot down so hard it sounds too strident, and it's always around that frequency. I picked that one up from Mike Minkler who uses one all the time on Foley. I'm obsessional about it, I can't stand esses.