Actor Jason Clarke couldn't believe the details surrounding U.S. politician Ted Kennedy's fatal 1969 car accident when he first read the script for biopic Chappaquiddick.

The Zero Dark Thirty star brings the former Senator to life onscreen in the movie, which centres on the events surrounding the tragedy, when the brother of late U.S. President John F. Kennedy drove his car off a bridge into a pond.

His passenger, Mary Jo Kopechne, drowned in the accident, but Kennedy didn't report it to police for several hours after returning home, sparking a major investigation.

Jason was unaware of the headline grabbing story before he was sent the script to play Kennedy, and he struggled to wrap his head around the strange turn of events, which resulted in Kennedy simply receiving a two-month suspended sentence for leaving the scene of an accident.

"I read the script on a plane and this conundrum of this man and how he came out from this early act here, of a woman dead in a car that he caused (sic), to being elected to the Senate again!" he recalled.

"I was like, 'Holy s**t, how does this happen?' I was very upset and it was a long flight. My wife asked me if I liked it and I didn't really know how to talk about it. I read it again off the plane and I was like, 'Come on, this is not real. This couldn't happen!'"

Clarke "went down the rabbit hole" as he researched the incident, but his findings only left him more perturbed about Kennedy's mindset at the time.

"I didn't know how to play this now," he shared. "How do I play a guy that does this? How does anyone watch that?"

Clarke came to realise Kennedy must have been mentally scarred from losing his older brothers in shocking assassinations - first John in 1963, and then Robert F. Kennedy in 1968, while he was running for the presidency.

"It's a very strange existence Ted has," he mused. "I think that (family tragedies) played a big part in why he walked away from the car and didn't report it. It gave me some context of the clinically depressed state that Ted was in, or at least an existential crisis. That's what made it so compelling for me."

The actor had to hire a dialect coach to help him perfect his accent as the late Massachusetts native, a task which was further complicated by the fake teeth he had to wear.

"My teeth are not like Ted's. I don't have that big a teeth; they're smaller and crooked so it was (tough) to get the teeth to fit in so that I wouldn't lisp or hurt, but also to get the size right...," he explained.

"The accent took me months of drills, and working with a dialect coach was so specific, and then to put the teeth in (took a lot of work) because I couldn't dentalise my T's for a long time!"

Kennedy died from brain cancer in 2009, aged 77.