Escort missions are the missions that you always have to protect an NPC. If the NPC dies, the mission'll be failed. However, there are many scenarios you might have encountered in this type of missions. And here are the details of some scenarios:
"Protect the Engineer while he repairs the car" is maybe the easiest scenario. The Engineer only stays at one place because he has to fix the car. However, this scenario is almost similar to a Defend mission but not one because the Engineer must stay alive.
"Help Smuggler loot the warehouse" is probably a hard problem for many players because 2 Panzers or 1 Scienfist and 1 Rager might spawn at the time. However, the player can kill them easily if they use the advantages of red barrels. After getting inside the warehouse, the Hero and the Smuggler will have to kill the trapped zombies until they find a truck full of whiskey crates. And the mission ends successfully.