Hubert Rees played a bit of a daft staff officer, Captain Ransom, in The War Games. It's a lovable performance, particularly when he excitedly tells Lady Jennifer the wonders of paperwork, thinking that she is as enthralled in the subject as he is. Rees's next role in Doctor Who was as Stevenson in The Seeds of Doom - a small performance, but one that is absolutely crucial to the success of this story.
Look how intently Rees as Stevenson examines the Krynoid pod that his fellow scientists pulled out of the ice at the Antarctic research base. He tries to put his finger on what is different about this frozen pod, and then decides: "It's alive. That's it. It's alive!" Look how Rees delivers that line. He doesn't shout it out as Archimedes would, but instead he almost mutters it under his breath with deadly seriousness. It's an acting decision that lends so much gravitas to what could have been a melodramatic moment, and it fittingly sets the scene for the rest of the story.
Speaking of fine performances (and I will be a few times over these next six episodes), Tom Baker continues his dark turn as The Doctor started in Pyramids of Mars, perfected in The Brain of Morbius, then, somehow, exceeding that perfection in this story. Never before has The Doctor appeared as churlish, almost downright rude, as Baker's Doctor in The Seeds of Doom. He balances his berating of the Antarctic base staff by being humorously eccentric in the scenes set in Dunbar's office, playing with his yo-yo, playfully plopping his golf shoe clad feet on his desk, and departing for Antarctica, toothbrush in hand, with the words "No touch pod. Could be dangerous.".
The transformation of Winlett from a normal human being to a grotesquely altered alien life form is done staggeringly well, with each revelation of how far his condition has deteriorated being even more shocking than the last. A fine set-up episode to what looks to be a monumental story.