What can Doctors fans expect from the 2,000th episode?
"Oh, wow - everything they wouldn't be expecting, really! It's way out there and it's really good. A regular Doctors fan will be riveted because it's got so much in it that they're not used to and they haven't come to expect from Doctors. It's extraordinary. I watched the episode recently at a cast screening and I thought it was amazing."
What can you tell us about the storyline?
"A potentially fatal virus is unleashed at The Mill, but it's all about what happens because of that and how each person is individually affected by that on all levels. It's all about how the staff at The Mill deal with it because they're locked in with the virus and they can't get out! I can't give too much away, but it's all about how they deal with what's going on and what happens to each of them as a result."
How does Julia react to the situation?
"Well, obviously Julia is the practice manager, so she takes control as Julia always does. However, something happens and then things go wrong… Again, I can't give too much away! (laughs)"
Was there an air of excitement around the set when you were filming this milestone for the show?
"There was! But with it being shot the way it was, and because there was so much packed into the episode, we tended to film in isolation - very much like the storyline in the episode. So I didn't tend to see the others doing their stuff - but then I wouldn't, because they weren't interacting with Julia. Many of the characters end up isolated in their various offices and in various other places in the building. So it was really strange - this extraordinary thing was being made, but you only really saw your immediate part."
It must have been nice to watch the finished product at the cast screening, then!
"Yeah, exactly - it was fantastic to see it all together. Obviously I knew the story because I'd read the script, but to see it all put together was fantastic and amazing. When you think what we do on a very small, limited budget on this show, to watch something like that come out is quite mind-blowing, really. It's great that we can turn out drama of this quality."
Were you glad to get the hour-long slot for the 2,000th episode?
"Yeah, definitely - it needed an hour to do the story justice. But when I watched it, it got to the end and I thought, 'Ah, no!' - because I wanted more. I wanted it to go on. You know when you go to a movie and it ends and you go, 'Oh, I wish it was about 20 minutes longer'? It was like that. It didn't feel like an hour at all, it just rips along."
What do you think is the secret of Doctors' success?
"Oh gosh, if I knew that, I'd be a multi-millionaire, making programmes all over the world and coining it in! (laughs) I do think that one of the main ingredients for it is that it always has excellent actors in it - all of the guest stars who come up to do Doctors are wonderful. It's very difficult for them because they hit the ground running, and at the beginning they look like startled rabbits in the headlights as we work very, very fast. But they come in and they turn out these wonderful performances and they're a joy to work with. I think that's true of the regular cast members as well - I think every single person in this show is the most consummate actor. You have to have that about you, because there's no time for rehearsal, so it has to be instinctive and you have to be terribly disciplined. But at the same time, we have such a laugh on set."
Do you think there's anything else that has made it so popular?
"From the public's point of view, it's the fact that we do have a serial element to it because of the regulars, but also that every single day is a complete story featuring the guest artists with a beginning, a middle and an end. So even if you haven't always caught up with the regulars, you do get a little mini-drama every day. I don't think anyone else does that, so we're crossing two boundaries by having a serial show and a complete drama within it every day."
Who have been your favourite guest artists to work with?
"Oh gosh, I've been here so long that it's hard to answer. We've had some wonderful, wonderful people - we've had Brian Blessed and many other extraordinary people who I'd watched for many years and admired. I also had a wonderful, wonderful episode with Eric Sykes - he was brilliant, and that was mainly him, myself and Christopher Timothy. I also enjoyed working with Richard Briers. It's just a joy to work with these people. It's also quite odd, because you work away here on the set, and eventually everyone seems to turn up to work with you, which is nice!"
As Julia is the longest-serving character, why do you think she's become so popular?
"I don't know - is she popular? If she is, I'm glad she is! I think with Julia, there's an element where you don't always know what you're going to get. People probably don't remember it, but I often think about a wonderful episode that I did with Barbara Dickson. She was lovely to work with and it was a brilliant storyline for me. Her and Julia were old friends, and as young girls in their early 20s, they were rock chicks following the rock bands around and were groupies, for want of a better word. So that tells you a lot about Julia's background - she was absolutely a wild child. I think she still has that wild streak in her, so whenever there's an opportunity in an episode - a party or if we go to a club - I let that come out in her. Also, although Julia is a stickler for work - and quite rightly so - she's got a heart of gold if anybody needs anything. So there's all those different levels to Julia that you get to see, and maybe that's why people like her. Maybe there's a bit of that in all of us!"
> Click here to read more from Diane Keen in Soap Scoop
Doctors' 2,000th episode airs on Wednesday at 1.45pm on BBC One.