The secret to that flexibility is being equipped. Before you take charge, you need to equip yourself with the knowledge necessary to handle the event. Learn what options are available for a successful event. Communicate well and frequently with people you will be working with so that all are clear on their expectations on the day of the event. If, despite your best efforts, your clients show up with a different agenda, make sure you have material available to you to adapt on the spot. Calmly step aside, reassess, come up with a quick plan based on what you know of people's expectations and the material you've brought with you, give yourself a rough outline to work from (even if only in your mind), and then lead the event calmly. So no signs of being flustered or nervous to your audience/clients. If you express confidence, take control, and see it through, no one will know what you are juggling to keep it all moving forward.
Events are have you walking a fine line. Your clients expect you to lead ... yet, they also expect you to be in accord with their wishes for the event if they have hired you. You need to guide them through the event, being respectful of their needs but tempering their enthusiasm with reality gently when needed. It is an art that takes some time to learn and longer to learn to improvise to meet the needs of the crowd in the moment.