In the beginning of Star Wars: The Force Awakens, we meet Finn
as a member of a raiding party of Stormtroopers that attacked and destroyed a village on Jakku, where Poe and BB-8 were obtaining a missing fragment of a map that would lead the Resistance to find Luke Skywalker.
Finn experienced an act of conscience due to that event.
Not agreeing with what the First Order did, he left his position, first trying to escape to the Outer Rim, then later defecting to the Resistance.
Towards the end of the film, Finn reveals that
he worked in sanitation while stationed at Starkiller Base, and that the village attack was his first real field combat.
So, my question is - how did Finn go from that position to field combat in a major operation to retrieve vital information, without seemingly doing anything in between? It seems too convenient for me, just a way to make sure he was involved in two key points of the story:
the first being the event that caused BB-8 to go on the run and eventually met up with Rey and later Finn. And the other event where Finn happened to know a way into Starkiller Base and knew someone who they could force to lower the shields protecting the super weapon.