I'd say the biggest mental changeup, is how to fit in with a large crew you are new too. Most of the time if you are brought in as a day player, the regular crew has probably worked together for years, and have a routine. NO two crews are alike, and one has to observe and figure out how you can fit in to the flow. First thing is that you have to observe the lighting set up, the rigging crew has already done a lot of the work setting up, and you have to size up what the shooting crews task at hand. Usually the most deceptive simple setups are the most complicated. Read the call sheet study how production has planned the day, how much they plan to do before lunch, most importantly especially if you are outside, observe the weather conditions, where the sun is. Almost think like the DP, who has a job to do , within a limited time. That way you can anticipate what is going to happen before the DP tells the gaffer and the key grip what he needs. Being observant and in position, is alike a basket ball game, where you know where to be on court during a fast break. Eventually you will figure out how to work with different crews and you will find you phone ringing for more days on set.