SINGAPORE - First-time director Dick Lee had a difficult time filming his new biopic film Wonder Boy as he was painfully reminded of his rebellious teenage past.

The 60-year-old singer-songwriter said he had to take breaks when shooting certain scenes because it reminded him how much of a "monster" he was to his family.

He even had to tell singer-actor Benjamin Kheng, who plays him in the movie, to dial up the meanness. "I hurt my family and I needed to tell Ben, who was a bit cautious, 'No, go all out. I was worse than that.' So it was hard for me to push him to be more violent and horrible to his family," he said at a press conference for the movie on Tuesday (Aug 1).

"Making him do that was hard because it reminded me how much I hurt them. That was pretty terrible."

This was made more difficult for him because his mother died in 2015, just a year before filming.

At the press event, Lee said to Constance Song, who plays his mother: "Every time I looked at you, it was very hard for me. It was still very raw. I want you to be exactly like her but, at the same time, I couldn't bear to see it."

For Kheng, 26, one of the attractions of the lead role was that he could live out his rebelliousness "vicariously through (Lee)".

"My sister was the rebellious one," he said, referring to his younger sister Narelle, who is in the local pop-rock band The Sam Willows along with him, Jonathan Chua and Sandra Riley Tang.

"I never had a rebellious phase. Due to some family difficulties and stuff, (I had) to grow up faster as the older child. Reality hit very hard for me and I never really had a chance to rebel."

Despite his lack of rebellion as a teenager, Lee said the casting of Kheng was a perfect fit because they are both musicians.

Lee said: "We share a lot of issues about being a musician in Singapore now, it's not very different struggling to get the audience. My main message was to keep going and keep believing."

Kheng agreed, adding that he identified with the obstacles his character faces as a local musician.

"What drew me to it was this struggle about long hair ban, rock music ban, just the refusal to play local music on air.

"Social media has really enabled everyone (today) and it's amazing to see how time has passed since then. It's really cool to look back on that; we really have come a long way. We've definitely got more to go, but it's nice to see how far we've come."

Wonder Boy opens in cinemas on Thursday.