Sidney is so much a tool for storytelling that I would liken him to punctuation. He exists to draw out the inner thoughts, fears and desires of our more three-dimensional characters, most notably Hawkeye but also Margaret, Klinger, Charles and the patients he treats on the show. Their psychoses are so often based in their fears, their denial, their disbelief, their unwillingness to take personal responsibility for their circumstances – which is not usually how real mental illnesses work, but still makes for good television. 

I’ve also used to term “Sidney ex machina” to describe his function:

Deus ex machina; plural: dei ex machina; English "god from the machine" is a plot device whereby a seemingly unsolvable problem in a story is suddenly or abruptly resolved by an unexpected and unlikely occurrence.

The 4077th will hit a wall with a patient (sometimes the patient is Hawkeye) that they cannot overcome, because they’re experiencing an illness of the mind and they don’t specialize in that type of illness - although you could make an argument for Hawkeye “therapizing” his friends (Margaret in Images, Radar in Hepatitis, BJ in Period of Adjustment) but he’s still not trained. When this happens, someone will go “get Sidney on the line” and every time without fail, Sidney successfully fixes the problem. This wouldn’t land so well if he was a recurring character on the show. 

Hawkeye is so in touch with the inner workings of his own mind and heart I wouldn’t necessarily put it past him to be able to monologue his way through his problems, coming to the solutions on his own (maybe with the exception of GFA or Bless you Hawkeye) but you still get the sense that he already knows the answer, he just needs someone to help draw it out of him. That’s Sidney’s role. He’s really just there for Hawkeye’s voice to have something to bounce off of so it becomes audible to himself and us, the audience. 

There’s one brief exception to Sidney being used this way and lol, it’s no surprise to me its in the Written-by-Alan-Alda Dear Sigmund. Alda’s episodes do tend to deal more with character drama, and I imagine he couldn’t resist taking a stab at Sidney. We learn that Sidney’s struggling with the loss of a patient – but only after Hawkeye and BJ read his private letters, really his journal, which is rude as fuck btw, but to me unintentionally emphasizes how much of a barrier there is between the audience and Sidney’s thoughts/feelings/fears/desires. But I can’t think of any other occasion where we get to see what’s beneath his calm, cool, professional exterior. 

There’s other times I was curious about that.. In War of Nerves he’s supposed to be at the 4077th as a patient, but he leaves the mess tent because he has a head injury that no one is considerate of, and he ends up treating people when he’s the one who’s supposed to be recovering. 

If you choose to see Hawkeye as getting progressively worse as the war wears on him (and idk if I do personally because the show is so episodic but that’s another post) then I have to wonder what it feels like for Sidney to have to keep treating him, especially in Goodbye Farewell Amen, where we finally see a crack in Sidney’s normally neutral expression, his consummate professionalism, as Hawkeye comes clean about what really happened on the bus. Like… they’re friends, it’s already ethically questionable to have Sidney treat him, and then we see exactly why that shouldn’t happen when Hawkeye is understandably upset that Sidney has decided to send him back to the 4077th. There is a moment of forgiveness and gratitude that passes between them in Sidney’s final scene in the series when Hawkeye thanks him, not insignificantly while he (Hawkeye) is performing surgery (to me it feels like a nice callback to OR), which he’d previously wondered aloud to Sidney whether or not he’d ever be able to return to. 

And here I am again saying that “flat” characters, of which Sidney is MASH’s best example, aren’t poorly written when they’re fulfilling their intended purpose, which Sidney does very well almost every time we see him. He’s so good at his job that it even feels weird for me to talk about his thoughts and feelings in fic, I want to get him in there, have him draw out the interiority of whichever character he’s in conversation with and then be like “glad we had this chat, peace” and actually that is how I see him being used pretty regularly in fic.