"When they heard this, they were furious and began shouting: 'Great is Artemis of the Ephesians!' Soon the whole city was in an uproar. The people seized Gaius and Aristarchus, Paul's traveling companions from Macedonia, and rushed as one man into the theater." NIV translation
Demetrius was successful in getting the workmen stirred up and causing a riot. While they were shouting to lift up their god, in their hearts was the issue of money. So, they seized Paul's companions and drug them into court. This is what Jesus was saying when He told us that we, as Christians would be persecuted. It happened in Ephesus with the early church; it happens in many countries today and it will continue to happen and get worse until Jesus comes back to get us.
"Paul wanted to appear before the crowd, but the disciples would not let him. Even some of the officials of the province, friends of Paul, sent him a message begging him not to venture into the theater." NIV translation
When Paul saw his companions being seized, he wanted to help defend them but those around him wanted to protect him. Whether Paul was correct in agreeing to stay away from the theater I do not know. Maybe God used those around him to preserve his life or maybe an opportunity to display God's power was missed. The ability to know God's will for us in any situation comes from the Holy Spirit.
"The assembly was in confusion: Some were shouting one thing, some another. Most of the people did not even know why they were there. The Jews pushed Alexander to the front, and some of the crowd shouted instructions to him. He motioned for silence in order to make a defense before the people. But when they realized he was a Jew, they all shouted in unison for about two hours: 'Great is Artemis of the Ephesians!'" NIV translation
The riot is happening in the theater and, as usually happens, many of the people involved didn't even know why they were there. The Jews pushed Alexander forward to speak on their behalf and to make it known that Paul and his followers were not a part of them. They did not want to get the blame for what Paul and the disciples were doing but, when the crowd found out that Alexander was a Jew, they shouted him down and did not listen to his defense of the Jews. It seems like this sort of riot happens even today when people stand up and try to tell the truth of Jesus Christ. The simple fact is that many men love the darkness and their homemade gods and they will try to drown out the message of salvation through Jesus Christ.