Nathan Fillion is one of the most successful actors in the entertainment industry today. From his days as Malcolm Reynolds in the sci-fi hit Firefly, to his current role as John Nolan in gimnow the ABC series The Rookie, Fillion has proven himself to be an incredibly versatile performer. But what is the secret behind his success? The first key to Fillion’s success is his willingness to take on a variety of roles. From his early days on One Life to Live, to his more recent roles in Castle and Modern Family, Fillion has never shied away from taking on…