I just finished tinker tailor soldier spy again. I've seen it before but I love the superb actors, subtlety, and mood of the entire film; it's only one of a few films I've ever bought.
One of the themes in the film is about doing one's duty. Prideaux did so and followed Control's orders to Budapest. Smiley did so, came back to find out the mole, and take over after Alleline left. Prideaux did by dispatching Hayden. Guillam did by rejecting his lover for his job.
What is to do one's duty? For me this means the performance of one's tasks, despite the personal consequences or feelings. It is the choice to do the appropriate or fair thing, above all else.
Doing one's duty is hardly an easy thing. Many of the characters in the film experience social loneliness in their lives, as shown in the ending. To choose an action that may stigmatize your social standing is something to be aware of.
I believe, however, that one's duty is important. For me, one's duty is one's higher calling in life. It is the thing a human being is supposed to do in life. As a human being, I would rather spend my finite time on earth on my higher calling.