I definitely agree not to over prepare, but I find it helpful for me to think a little on the different paths they might take and have some stuff constructed so that when they inevitably do something you don’t expect, you have something close enough that you can repurpose or guide them back to a prepared path. It depends on what you’re good at. If I had to completely improv a session, I’m positive it would be rough. Others could improv entire campaigns without their players noticing.