Yeah, I thought that the story was Lord & Miller were having issues with the shoot and doing a TON of takes per shot (figuring they'd fix it in the editing room) and then it just wasn't coming together and the dailies weren't wowing the producers. My guess on that is that the tone and style they were going with didn't jive with what the producers had in mind. Obviously, they wanted something a little freewheeling and comedic, but maybe they got more than they bargained for, and brought Ron in to clean it up and pull out the performances they were hoping for.
I'm guessing that L&M do fine with comedians trained in improv, especially folks who come out of the major improv houses in the U.S. (e.g., Groundlings, UCB, Second City), but with actors who are more traditional and may have only had a few improv classes, it's not much to go on. Improv is just...different from traditional acting. I think you can take comedic actors, including those grounded in improv, and they can often do serious work, but it's a lot harder to go in the other direction, and I expect it's really hard to change gears on the fly within a single story.