Are there certain times you can just hold "Y" to pull up the screen? I've tried holding it down but it just swaps people and then nothing.
I think you automatically get free select when you finish the story, but I haven't beaten it yet so I am not positive on that. Otherwise, when you are between missions, go to one of the SHIELD pads and activate it, then switch to whatever character you want. After that, you can freely select characters with Y whenever you want, at least until you start the next mission. Then just repeat the process.
He who does not punish evil commands it to be done. -Leonardo da Vinci