Your express passes can be scanned at the beginning of the line or some can be scanned somewhere in the line. If it's scanned in the beginning you have nothing else to do. If it's scanned in the middle you have to show it at the express entrance to gain access.
There are only a few rides that you merge with the regular line.
Hulk
Dragons
These are the only ones that I can think of. The main reason for Hulk was people were complaining that they could not have a choice to ride front. The old express can up the back by single rider. I wish it still was that way, it's the most awkward feeling this way. Depending on TM scanning EP's they might stop the regular line to merge or not. Either way the regular line people normally are not happy. They need to fix this, if you want front wait in the regular line period.