As an example: You are on a charter boat in Hawaii, and they tell you they're going to do a drift dive off Red Hill. It's the second dive, so they tell you to watch your computer carefully so you don't go into deco. You jump in the water and follow the divemaster. The current starts to push the group apart, and you're anxious, because you don't really know where the guide is going. You don't know how long the dive is expected to last. You're watching your gas and your no-deco time to see if either is getting low.
The same dive, done differently: It's a drift dive off Red Hill. You ask the DM how deep he expects to go, and he says 60 feet. You plug in 60 feet in your dive computer's planning mode, and find out you have 45 minutes at that depth, given your previous dive. You check your tank, which is full, and you calculate your gas consumption at 3 ATA for 45 minutes and discover that, with a novice diver's gas consumption, you are woefully short of gas for that dive. You talk to the DM and ask him how long he expects the dive to run, and whether you will be at 60 feet the whole time, or if you will have an option to move shallower. You match his answers to your gas supply and figure out what you can do. You look along the reef and check your compass, so you know roughly what direction the dive will go; that way, if you get separated from the guide, you and your buddy can continue in the same direction.
See the difference? Thought given to the dive before you get in the water prevents problems. Discussing the dive with your buddy and clarifying a plan and signals and contingency procedures prevents misunderstanding and stress. The more you can do ahead of time, the smoother and more relaxed your dive will be.