The BBC medical drama reaches its milestone tomorrow afternoon, marking the occasion with an hour-long special which sees a deadly virus unleashed at The Mill.
Speaking to Digital Spy about what viewers can expect from the episode, Keen - who plays long-standing character Julia Parsons - commented: "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."
She continued: "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."
Doctors' 2,000th episode - titled 'Quarantine' - airs tomorrow at 1.45pm on BBC One.
> Click here to read our interview with Diane Keen
> Click here to read more from Diane Keen in Soap Scoop