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The best way to illuminate a boundary is to cross it. Barring that, the second best way is almost cross it. Have your characters accomplish some major task together, despite long odds. Then, a...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/35906 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
The best way to illuminate a boundary is to cross it. Barring that, the second best way is almost cross it. Have your characters accomplish some major task together, despite long odds. Then, as they are celebrating that small victory, have them catch each others eyes. Hold them there, teetering on the brink of it becoming something greater, then in perfect synchronization, as if sharing a well established joke, have them smile with mischief shining from their eyes. Both confident in the support and loyalty of the other. Both thrilled with what together they have accomplished. Both knowing that their current lifelong bond is better than the steamy short-lived alternative which awaits them down the alternative path. You don't have to say any of it. Just make sure to show that each is happy in the now and untroubled by their aborted brush with intimacy. Romantics will always hope that this will change, but if the friendship is unfailing and supportive in both directions, it will fill the "better than real life" fantasy that your readers crave, without getting all mushy.