Yes, I wondered about Mackenzie Cook for Nobby (actually, I went a step further and fanarted it, see http://nestofstraightlines.tumblr.com/post/39629665669/watch-tv-series-poster-just-for-pretendies
). He's got the right vibe for Nobby, even if he is a bit tall, and can deliver a very funny irritating/endearing performance. As theat drabble demonstrates, I'm a bit obsessed with the Watch TV series idea and what I do when I'm obsessed with something is work out how I would do it (tot he point of a half-formed script) - how you could translate a book that has a whole hierarchy of contexts surrounding it (Watch sub-series - Discworld - comedy - fantasy etc) into an accessible TV show. What's the hook you use to convey the important stuff to the audience? I can't wait to see what they do with the Watch.
But I wouldn't choose a Watch story for a big stand-alone film, for exactly that reason. I'd go for something like Moving Pictures. Actually yes - definitely Moving Pictures. It's parody angle gives people an immediately understandable hook for audiences to be drawn in by, and you can start to build the film version of the Disc on the back of that. The characters are good but not really the focus, so you don't have to worry about taking a few liberties. And the fact that it is parodying and discussing the very medium you're translating it to gives it an immediacy.
Either that or Wyrd Sisters, which happens in a setting that doesn't wear its fantasy elements so obviously on its sleeve. Lancre is a gentle easing into Discworld that A-M because it's kind of folkloric/Shakespearean thing we all have a cultural understanding of without having to get to grips with the wider Discworld. And if you're got the Shakespearean plot going on too, it makes things all the clearer.
Basically, with Discowrld adaptations, I think the grail is accessibility for a new audience!