What if they interview people from one side and none from the other? Would that be biased or not?

Check whether the author uses the interview as evidence to favor one side. If yes, it would probably be considered bias but that depends on the context. If no, the interview is just being used to inform the reader more (not considered bias). Also, be aware that balance is not achieved by digging up an extreme fringe group to state their case as some counter-balance to the wider majority and minority insights into an issue -- that would be trying to manufacture a balance that suggests the extremists are legitimate in their fringe beliefs or opinions when few people would consider them reliable.