Closest and Heart-to-heart

Closest adverb - (superlative of `near' or `close') within the shortest distance.

Heart-to-heart and closest are semantically related. In some cases you can use "Heart-to-heart" instead an adjective "Closest".

Heart-to-heart adjective - Openly straightforward and direct without reserve or secretiveness.

Closest and heart-to-heart are semantically related. Sometimes you can replace term "Closest" with "Heart-to-heart", this terms are similar.