A while ago I wrote about the modern companions’ reactions to regeneration. While Rose and Clara were far from the first, they belong to a small but select list of those privileged – or ill-fated – to witness the Doctor’s demise:
Ben and Polly witnessed the very first regeneration. Like the audience they represented, they watched in amazement as their old friend faded away, to be replaced by a dark-haired bohemian. Unfortunately the following episode is missing, so their response is lost to posterity.
Sarah Jane Smith witnesses the Third Doctor’s transformation. Her friend the Brigadier has some idea of what to expect, having finally accepted that the Second and Third Doctors are in fact the same man. In addition, the Doctor’s associate K’anpo appears in his final moments to prepare her for what’s to come. Like Rose later, she is first baffled and then amused by her old friend’s new ways, and quickly grows close to him again.
Also like Rose – and unlike any other classic companion – Sarah Jane is home and among friends when her Doctor fails her. She and UNIT do their best to face danger without him. And as he always does, the Doctor recovers just in time to step in and save the day – though not without significant help from Sarah Jane.
Adric, Nyssa, and Tegan attend the Fourth Doctor’s passing. None of them has traveled with him for very long, and none has any idea what to expect. Nyssa and Tegan do their best to look after the baffled young man who appears in his place, but with Adric captured and the world where they land not quite what it seems, their difficulties only increase with each passing moment. Fortunately the Doctor recovers in time to rescue Adric and extract the TARDIS from the Master’s manufactured world, and the four travelers soon become fast friends.
Peri has the worst time of it: only Six ever attempted to murder a companion while recovering from regeneration. Still, the two seem to more or less get over it, and continue to travel together for some time. I haven’t seen the episode, so I can’t say much about her response.
Mel is present for her Doctor’s death, but unconscious, and the two are quickly separated. The Rani adds to the confusion with her disguise. When the two old friends finally meet, each thinks the other an imposter. But her single heart and his aversion to carrot juice soon set the record straight; once he explains his amnesia, the two get on with defeating the Rani. Mel claims to know something of regeneration, and has little trouble adapting once she’s sure the Doctor is the Doctor.