Alps plane crash: How to keep someone out of the cockpit

Did a Germanwings co-pilot deliberately keep his colleague out of the cockpit in order to crash Flight 4U 9525 into the French Alps?

A prosecutor at the heart of the investigation believes recordings indicate that the pilot who left the cockpit was prevented from getting back inside.

How is it possible to keep someone out of the cockpit? The BBC explains the protocol flight crew go through to enter the cabin, and how someone can be kept out.

Video footage from an Airbus promotional video