It's okay that it wasn't a lead role. Perhaps it was more successful because he wasn't the lead.
MBLAQ's G.O. attempted his first acting gig on SBS "Ghost" as the youngest detective on the Cyber Crime Team.
"Ghost" drove audiences to panic every night, making drama out of multiple twists and turns in the stories. There were episodes where the lead roles didn't even appear for 10 minutes in an episode and there were episodes where smaller characters were given the full 72 minutes.
Lee Tae Gyun (G.O.'s character) appeared to be a supporting role at the beginning but as he started to be suspected of being a spy in the police department. However, the writer wrote him down as his presence started to become too big and he remained a supporting role for the rest of the series.
However, on the 19th episode that aired on Wednesday, his presence was increased yet again. He found out who the spy was within the police department and found proof that the head siphoned money from the public funds.
G.O. benefited from these changes in appearances. Instead of being a strange middle character, he was able to leave more of an impression on audiences in short bursts. For the actor himself, he was able to focus on the acting at short periods at a time which was better because he was under less pressure.
Idol members have been known to take lead roles without actually having been confirmed as an actor. This is the fault of the crew who are only keen on popularity but they are often criticized for the lack of acting ability in the cast. In this manner, G.O. was wise because he didn't have to face the trouble of keeping the series afloat but could appear and impress in strong and short bursts.
"Ghost" only has the finale to air.