I thought his answer was interesting enough that I transcribed Julian Fellowes' words for you, Jules, and for anyone else having trouble viewing the video:
Interviewer: "Julian, you've produced and written movies and drama not just in the UK but elsewhere. Do you think this is a particularly propitious place to make good fiction?"
Julian Fellowes: "Well, I mean yes, because I think the proof of the pudding is in the eating, and the fact is we make very good drama. I do think one thing actually and that is: period drama--which I don't work in exclusively actually, contrary to what the papers say, but I do do a lot of period drama--and I think our actors have a kind of understanding of period. For Europeans, the past is in them as well as the present, and I think they are at ease in that genre in a way that the Americans, for instance, I think find harder. I think Americans are wonderful film actors, wonderful film actors--best in the world. But they are a very contemporary race and they look forward all the time, and there is something about period drama where they tend to go into a strange place called "Period" where people wear funny clothes. Whereas, I don't think our actors do that; I think they make it very real. And that is, of course, with something like [what]we're doing, very, very helpful. I mean, the cast is so much the main reason for its success."
To me, the interesting thing about this quote is that what Downton Abbey most reminds me of are the great American-made TV miniseries of 30 to 40 years ago--Roots, The Thorn Birds; Rich Man, Poor Man; The Winds of War, for example--more so than the great British-made series of that era such as Upstairs, Downstairs; Poldark, or The Six Wives of Henry VIII.
But as far as generalizations go, I guess Mr Fellowes words are pretty much on the mark. It's definitely true of a certain British gentleman of our "acquaintance," who looks very real and very much at home in whatever time period you set him.