Do Mandarin and Cantonese lie on a "spectrum" of mutual intelligibility?

[miconazole]

Here's the thing, a few years ago I caught a little bit of a conversation in a movie where two characters were speaking, I think, Cantonese (I mean I have no way to go back and verify or anything) but a third was speaking what seemed to me like perfectly intelligible if heavily southern-accented Mandarin. There were no subtitles or anything, so either the directors expected their audience to be bilingual, or the Mandarin-speaker was supposed to be intelligible to Cantonese-speaking viewers... ? As an aside, she was telling them how to artificially inseminate their pig or something, I dunno.

I've NEVER read anything about this being the case, but can a native Cantonese speaker understand a sufficiently southern dialect of Mandarin, without separately knowing Mandarin?