A few missions in the game I talked to a guy that wasn't mentioned in one of the hurried briefings, and found the talk function suddenly taken away. It turns out that it was a completely new mission!

Being completely confused as to what the hell was going on, I looked up a walkthrough, and apparently the game has missions that aren't part of the main storyline! Is there any way to have any kind of control over what is going on without this crossing my fingers crapshoot, or do I need to have a walkthrough babysitting to make sure I don't miss out?

For all who don´t want to wonder if they missed anything.

There are 8 optional missions.

Look at this guide on steam on how to get all optional missions:
steamcommunity.com/sharedfiles/filedetails/?id=648392014