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You are letting the characters develop naturally, according to what feels right for them, rather than forcing them to conform to an abstract plot point. This is good, of course. Now, if you really ...
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#4: Attribution notice removed
Source: https://writers.stackexchange.com/a/46920 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#3: Attribution notice added
Source: https://writers.stackexchange.com/a/46920 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
You are letting the characters develop naturally, according to what feels right for them, rather than forcing them to conform to an abstract plot point. This is good, of course. Now, if you really need to stop your characters going in a particular direction, I don't think you should abandon your good method and switch to the bad one (forcing the plot onto the characters). I think you might find it suprisingly effective to let the characters themselves guide you in the right direction. For example, consider the following exploratory writing process (not necessariy to produce the final draft, but to find solutions that you will then re-write): step (1), force the plot point onto the characters--even though they are developing such a strong and intimate relationship, romance is NOT developing; step (2), hang a lantern on it--e.g. have some other character urge the protagonist to start a relationship with the nurse ("Man, you two are _made_ for each other, you should make a move" "I get that it might seem that way, and you know, I do love her as a friend, but I just can't see her that way"), step (3), have the two main characters themselves work through the issue, within the action of the story--they talk about it, admit to each other that their life would be so much impoverished without their friendship, and yet they agree that they feel no romantic interest, perhaps discovering this about each other to their mutual relief... And now they can actually have a conversation about it, talking about friendship and love and their feelings about her fiance and his lost family etc. And, quite likely, even though you, in the abstract, aren't sure what would stop the characters from becoming romantically involved, now, through the eyes of the characters, you can explain why this is the case most conclusively.