Much of the advice you hear on the Web about interviewing and getting a job comes from an old Puritan ethic in American History. It is now false today and has been for some time. The advice of the past can be summarized as follows: put up and maintain a false face so as to avoid reprisal and gain reward for appropriate social behavior. The good behavior should always be there but straining to present yourself in the image of perfection only sets you up for problems later when your natural self is shown. If you do not normally speak with perfect grammar, then don’t speak with perfect grammar. Speak the way you actually are so you are consistent from interview to work day. Natural people do not really like fake people so don’t be fake but do strive to be good always.
Consider that a fake is always found out. If the real you does not fit with the company you keep, you will find yourself in a dismissal sooner rather than later. Instead of simply being passed over for a job, now you have a short stay to explain. Better to hold out for the kind of group with whom you will best fit with. It is said, you have to work with people you don’t like. Sometimes that is true but most environments have a natural way of sifting out those who no longer align. Anyway, authenticity is a choice. Often the easiest to make.