Any tips on writing a childhood friendship that as the characters age turns into something more?
Creating Romantic Feelings Between Characters might help! I’m pretty sure that the link mentions how the criteria should exist in some basic form for friendship, but romance is seen as a step toward a different kind of intimacy. So what I’d do is introduce a level of attraction that subtlety creeps up on the characters, since they’re already close.
Budding romantic relationships with friends can be shown through the small things that a character notices. It’s not rare for people to develop feelings for friends, but it’s often hard to pin down early on and it can get confusing. A character may just find that they really like spending time with someone more, or they can realize they feel a little anxious about something that used to not make them anxious. An example of this is accidental touches. You can bump into a friend and get over it pretty quickly, but if you have budding feelings for someone then you might be a little more stunned by it.
Different people also notice feelings at different stages. One character may realize right away and hide it because they don’t want to lose a friend, while another might be more oblivious and not realize what the physical signs are pointing to. How it unfolds really depends on the characters involved and how the plot and setting interact with them.
It helps if you can put them in situations where the attraction can really be shown or increased, or parallel a past situation with the new attraction so the reader can see the change. It’s even okay for a character to internally comment on the attraction, or talk about it with someone else, as long as you keep the thought from taking over into too much “telling”.
Also, people grow up, and part of growing up is changing. Their friendship may have been built on one thing, but they aren’t the same people they were as children and that alone can sometimes lead to attraction. You can include the past friendship as a good foundation, but the romance shouldn’t be built on the exact same elements as the plain friendship because there should be a reason the romance is only coming out now.
Different Ways Characters Can Show Love is another post I can offer to you for looking into the details of how each character may behave beyond the friend boundary. Their history, upbringing, personality, all of that mixes together to affect how people interact and form relationships. Like I said before, there is definitely a level of love between friends, so there needs to be some form of attraction that pushes them beyond that boundary.
Good luck with your story!