In dramas guest arcs have been re-defined, in the past it was those typical one shot hospital visits, or the criminal guest star on a Law & Order type show that took up most of the spots in these categories. The Academy of Television Arts and Sciences which oversee the Emmy Awards wisely changed this rule, because in both comedic and dramatic television series the "guest star" role has evolved into a a more complicated character along for the ride of the show.
For the sake of the award show purposes sometimes category placement is in the semantics, like Orange is New Black's Uzo Aduba, and Laverne Cox who were not placed in the main credits for the series' first season. This means both are considered guest stars for their work. By the way Orange is the New Black should be in the drama categories, but it will and can probably do better in the comedy races, but that's award show bias, I digress. Both Aduba and Cox gave some of the strongest performances in season one of Black and are incredible worthy of nominations at the Emmy Awards.
Then there are characters who have story arcs like Beau Bridges, Allison Janney, and Julianne Nicholson who clearly have story arcs that will begin and end. While most of these characters will probably return for episodes in the second season there will be an end to their stories, and this is clear through the way in which the writers have set up their characters. These are probably three of the strongest guest performances in years, and this show has perfectly intertwined their stories into the show.
A great guest star can sometimes steal the spotlight from the lead, these men and women are often scene stealers like Rigg, Preston, and Fox, but at the end of the day their lights never outshine the stars, and they bring a perfect balance to their show.
Outstanding Guest Actor in a Drama Series
Beau Bridges-Masters of Sex
Reg E. Cathey-House of Cards
Michael J. Fox-The Good Wife
Harry Hamlin-Mad Men
Pedro Pascal-Game of Thrones
Outstanding Guest Actress in a Drama Series
Uzo Aduba-Orange is the New Black
Laverne Cox-Orange is the New Black
Allison Janney-Masters of Sex*
Julianne Nicholson-Masters of Sex
Carrie Preston-The Good Wife
Diana Rigg-Game of Thrones