It's easy to see why Alan Tilvern had such a long and storied career as an actor watching this episode. The part isn't anything particularly special; he's a silver-tongued executive with a ruthless streak who turns to murder out of desperation, your basic villain in a suit. But Tilvern uses his expressive eyebrows and silky voice to become something more saturnine and sinister. I love the implication that he knew the story he told Smithers wouldn't hold up--the way Tilvern manipulates him first into accepting and finally into assisting with the cover-up is acted brilliantly by Tilvern. (Reginald Barratt as Smithers perhaps overacts just a bit, but he's got to sell the idea of someone so obsessed with DN6 that he'd go along with concealing a murder with very little persuasion. A little overacting helps in that department.)
The Hitchockian drama of Forester drawing Smithers into his web of deceit and murder is really the best part of the episode. Everything else is an exercise in contrivance, from "We can't go back the way we came, cats exist!" to "There's a person coming, scatter in every random direction and panic!" to "Hmm, grains of wheat covered with a sticky substance!" "Huh? Sorry, I don't really pay much attention to anything you say or do, Barbara. By the way, don't touch that wheat, it's covered in insecticide." It's all pretty much just getting everyone into the position of maximum peril for the next episode, which is kind of a shame because it doesn't really make nearly as much use of the 'miniaturized' concept as it could.
One thing that did strike me, though, is that this is the first time we've really seen the main characters split up along the lines that you'd think would be the norm for the series--Ian/Barbara, and the Doctor/Susan. It's the obvious way to group them, the aliens who've been traveling for years and the two teachers along for the ride, and yet it's just about the least common pairing they've done. 'The Daleks' has them split up like this in the last two episodes, and there are times when it's a pair and two singlets, but really it's only in the last story before Susan's departure that we see her really spending any time with just her grandfather. The actors play it well, given that it's a fairly perfunctory scene, but it's notable that the dynamic among the regulars has done so well for so long by resisting the "easiest" pairings.
Oh yes, and while I grumble about everyone doing nothing but peril monkey schtick in this episode, I have to love the dramatic touch of closing on the sink at regular size as the water slowly drains out. It's deliciously incongruous (as is the simultaneously tense and hilarious musical sting accompanying, "There's a sink in the lab") and leaves you to imagine the danger rather than attempting to show it.