The role of interpersonal communication is different between two people who are friends versus two people who are involved in a romantic relationship, but not as much as one would think. I think we can all recall a story of either ourselves or someone we know starting off with someone as a friend who later became a lover by one or more of what out calls “conditions for intimacy” like close physical proximity or shared episodes.
We hear a lot about intimacy, and I think that is the main differentiating factor between a lover and a friend. Two people in a very close friendship may experience a certain feeling of closeness and a strong desire for close physical proximity to one another, but only when two people become romantically involved does that need grow so strong that occupying the same space as the other person becomes the most important thing to them.