Writers as Actors
An actor's job is to take on different characters, according to the script that he or she's currently performing. However, some consistent traits of the individual actor also shine through, no matter what the role. A good writer also takes on different characters, or writing personas, depending on the situation; still, we can recognize the writer's individual voice within those personas.
How do you decide what role you need to play? Two major considerations are your audience and the type of paper you're writing.