Direct communication between two colleagues.
As often as necessary
Sitting together on a regular basis helps people stay informed and build a relationship of trust.
Ben Horowitz accurately describes 1:1 meetings as the “free-form meeting for all the pressing issues, brilliant ideas, and chronic frustrations that do not fit neatly into status reports, email and other less personal and intimate mechanisms.”
There are a couple of reasons why the one-on-one format is so valuable. It helps building a trusting relationship, staying informed and aligned, providing mutual feedback to help each other to grow, and addressing topics prone to getting lost in the shuffle (e.g. career development).