phrase

In police work, when a certain crime has been committed, often the police will question people who have previously been convicted of similar crimes. "Round up the usual suspects" means getting those people who have committed similar crimes before to the police station, so that they can be questioned.

The police work interpretation would be literal. On a more figurative level, it's also used to refer to those who are of a like mind, are predictable, and are expected to express certain opinions or behave in a certain manner -- for example, when the topic turns to politics, religion, and other controversial issues.