Really, if you aren't the absolute first people in line, your fastpass will be further back than the 11:45 time. The fastpass that you get will tell you exactly when to get in line for the autographs. They were also allowing people to choose from the 1st and 3rd sessions, but I am not sure if they will continue that. Once the 1st session is full, the 2nd session opens.
You can also do standby with fastpasses, but that is more of a hit or miss thing. Also, just so you know, it is one fastpass per ticket per person standing in line. One person cannot get the passes for the whole family. They all have to be in line to get the passes.