It is both the big things and the tiny things,
Take who I consider my best friend Vallie, we have a number of the same like, and a number of things we conflict in choice.
Conflicting choices are important too as it helps you see things differently. She is there for me as I am for her. We are approved to make medical decisions in case of emergencies. (other then husbands, we don't have a ton of family in the area)
On the other hand take my best friend E. E passed away 10 years ago. I do still consider him my best friend, because he is gone doesn't change that, my friendship with other people don't change that.
Not all friendships are good, sometimes people are not what they seem to be and its a growing experience and even that is valuable.