"Is there anything else that puzzles you?"
— Sherlock Holmes, A Study In Scarlet
Dirk: I need you, MacDuff!
Richard: You need me?
Dirk: No... That is to say, you are the grit around which the pearl of truth often forms. Your plain-speaking everyman perspective is useful counterpoint to... And you're a very good sounding board for my more outlandish theories...
Richard: You mean you think better when you're showing off?
Dirk: See? There you go. Cutting straight to the heart of the matter.
And Twilight will be my lowly assistant who asks silly questions with obvious answers.
[P]lot-wise, the companion’s job is fairly straightforward. They’re supposed to act in a particular way within the narrative. In that regard they’re fairly interchangeable. Most basic plots would work with any companion. Nothing about what Jo Grant does is that noticeably different from what Mel or Rose or even Romana would do in a similar situation. Yes, the flavor of it would change - the dialogue changes, and some of the mechanisms by which characters accomplish things change, but the basic shape they fill in a plot is the same. Which leads to a sort of diminishing returns - all you can do with them is switch the iconography around. A knife-wielding savage! A bossy Australian stewardess! Bonnie Langford!