In Jane Eyre, what is the significance of St. John Rivers, and how does the character affect action, theme or development of other...

This is a very interesting question, because, to my mind,
no examination of St. John Rivers would be complete without also comparing him to his
foil in the novel - Edward Rochester, therefore, if you don't mind, I will compare and
contrast both of these characters. You had placed them as tags in your question, so I
think this is what you wanted to know about.

To answer this
question you need to be aware of what these two characters symbolically represent in the
novel as a whole, and how they reflect a conflict that is played out within Jane
herself. One of the key points of dialectical tension in the novel is the struggle or
conflict between passion and sense, between emotions and duty. Various characters
reflect either one of these extremes to varying extents. Thus, for example, Helen Burns
is an extreme of sense and duty, a model of restrained emotion, whereas Mrs. Rochester
(the first) is an example of passion run amok.

By this way
of looking at the novel we can see that Rochester can represent passion and some of its
excesses, whilst St. John represents sense and duty. For example, Rochester as a Byronic
hero is a character who has quite a colourful past - he has had a string of mistresses
and is obviously sexually experienced. Thus when Bertha Mason burns his bed it is
symbolic of passionate excess on his part.

On the contrary,
St. John Rivers is excessively fixated on duty. So much so that he forsakes the woman he
loves to pursue his dream of going to India and working there as a missionary. It is
interesting to focus on how St. John is described. The word "marble" is used frequently,
as are other descriptions focussing on cold imagery:

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...the cloak that covered his tall figure all
white as a glacier.

Such
descriptions reinforce the coldness of his personality - he is duty taken to excess,
just as Rochester is passion personified.

Of course, what
you will want to think about in your essay is how these two opposing forces play out and
demonstrate the conflict that goes on within Jane herself as she wavers between these
two emotions. She recognises the danger in both, which is why she rejects becoming
Rochester's mistress and rejects marrying St. John for duty alone. It is highly telling
that she only married Rochester once he has gone through a purgatorial experience and
has been maimed. Thornfield could be said to be a symbol of Rochester himself, so with
its destruction we see a much more tamed Rochester at the end of the novel (he is
compared to a caged eagle with its wings clipped), and thus is ready to marry Jane, who
in so doing manages to finally reconcile the fight between passion and duty in
herself.

I have provided just one or two quotes, however it
would be well worth reading over the sections where these characters appear again to
gain some more quotes you could use in your work. Good luck and I hope this
helps!