Seo In Guk did a very convincing job of playing the goofy high school student pretending to be a corporate executive in "High School King of Manners." He did such a good job that when you finally meet his coldhearted older brother, also played by Seo In Guk, it's easy to believe that he is played by another actor. His body language is different. He speaks differently. Even his face seems different. Seo In Guk's eyes are bright and expressive when he plays Min Suk but flat and emotionless when the actor plays his older brother.
Seo In Guk nailed both roles but he is not the first actor to manage two roles in the same drama.
In "Phantom," So Ji Sub plays the police detective Kim Woo Hyun. When he dies in a fire, an avenging hacker named Park Gi Young is badly burned and has plastic surgery to look like Kim Woo Hyun. Then he takes his place. So Ji Sub has to play the detective to start with and then the hacker pretending to be the detective. The two characters were friends so Park Gi Young knows a lot about the man he impersonates. Still, there's a discernible difference.
In "Love Rain" Yoona and Jang Geun Suk play the parents of their characters as college students and then play their own characters doomed to suffer from the mistakes of the previous generation. It's fun to see them dressed in the clothes of another generation and then again as very modern lovers.
In "Bride of the Century" Yang Jin Sung also has two roles. She plays Jang Yi Kyung, a spoiled rich girl and Na Doo Rim, a plucky girl with little but a great personality. Na Doo Rim pretends to be Jang Yi Kyung to protect a marriage arrangement but her fiancé, played by Lee Hong Ki, falls in love with the substitute girl. Yang Jin Sung did a good job switching from cool disdain to perky optimism.
Park Shin Hye also played two characters in "You're Beautiful." For the most part she is only pretending to be her twin brother by dressing and acting as he might in front of people who never met him. For a drama minute she actually has to pretend to be her brother at the airport, complete with someone else's muscles. But mostly she just looks like Park Shin Hye very convincingly and attractively dressing and acting like a boy.
Why act a double role? Perhaps for actors it's a challenge and thus twice as much fun.
