What is happening in this moment and how does each character have a part to play (actions, incoming information, specific roles, someone to save, someone to fear)
What is the planned order of events if the characters don't intervene?
What changes when they do?
Treat each event like a mini-game and jump between them if need be. Don't be afraid to introduce something new in order to create a purpose for each character, but always consider what will happen if they succeed or or they fail. Try to balance these events so they don't get bogged down - always a concern when combat begins! Aiming for each event to take no more than a few rounds is a good platform to build from, but will also help you make it clear when your players are in much bigger trouble than they think they are. If they have been used to solving everything quickly but suddenly they find it taking longer, they should figure out the stakes have been raised!