Actor Lee Tae Sung became friends with JYJ singer and actor Park Yoo Chun when the two played time-traveling enemies in "Rootop Prince."

"I became close to Yoochun gradually," said Lee. "But it was as if I had always known him."

They grew closer when Lee consoled Park after his father's death, then they shared stories of common experiences such as spending time in Japan.

"It's not always easy to make friends on a drama set and even if you do, it's not always easy to keep a friendship going," said Lee.

But it was different with Park.

"We only have to say a few words to each other and we know exactly what the other person is thinking," said Lee Tae Sung.

Which other actors are close?

Song Joong Ki and Lee Kwang Soo became friends because they appeared together on the SBS show "Running Man" and in the drama "Nice Guy." Although they bicker on "Running Man," they are genuinely fond of each other. Lee Kwang Soo is the only friend who has ever been invited to Song's home. Lee recently commented on Twitter that he was concerned about his friend, who is serving his military duty, and that he wants to visit him soon.

Jung Il Woo is friends with both Lee Min Ho and Kim Bum. Jung Il Woo and Lee Min Ho attended high school together but grew closer after they were in a serious car accident.

"We've known each other since we were kids," said Jung of Lee Min Ho. "But then we were in a life-and-death situation together, so now we're friends who've hovered close to the afterlife together."

Jung considers his "Unstoppable High Kick" co-star Kim Bum to be his brother.

"When I was having rough times, he was there for me, and when he had troubles, I was there for him," said Jung. "We've known each other for five years now. He's a really dear friend."

Kim Woo Bin and Lee Jong Suk shared a bromance on "School 2013" and remained such good friends after the drama ended that they sparked dating rumors. Lee Jong Suk has said that Kim Woo Bin helped him deal with some of his stage fright.

"When people look at me and scream, sometimes I get scared. Then I just watch my friend Kim Woo Bin next to me and it helps me deal with it," said Lee Jng Suk.

So Ji Sub, the star of "The Master's Sun" says Song Sung Hoon, star of "When A Man" has been his closest friend since he debuted.

"I will like the guy as long as I live," said So. "I will always remember him helping me when I was distressed. I will always be good to him. He is a good brother, colleague and actor."