First of all, I never robbed a bank. Nor have I ever been in a car full of people who were planning to rob a bank. Thus, I have never heard their conversation, which makes it impossible for me to verify that this kind of talk is "perfectly natural to use in this context" or "phrased naturally."
But let's change the context to something I am more liable to witness or participate in, such as cleaning up a store after a breakin with a smashed-in front window. There's lots of glass so we know the cleaning job will be challenging. The window has been boarded up so we think we will at least be secure from the bad guys and weather. Then the man in charge says:
"I want two people to guard the doors and keep out customers." The man in charge points at people and orders, "You go to the front door and you go to the back door."
Insert conversation in question:
One of them says:
This is not what I signed up for!
To which the man in charge responds:
You and your friend signed up to do exactly as I tell you to.
Yes, I think that is natural and correct, especially if the situation is rather tense and I think it might be rather tense, either for cleaning up a broken-in store or robbing a bank. The man in charge might say "what I tell you to" rather than "as I tell you to." However, that might simply be a regional difference in language, pertinent to North America. It is my opinion that your story is good to go even though I have never participated in bank robberies.