The filmmaker continued that he explained to the Inception actor, how important the film is for his filmmaking career, and by the time the actor will be done with the film, he will be doing post-production, hence he will not mess up the film by making the wrong casting decision.
“Come on. This is a giant movie that is going to take two years of my life, and you’ll be gone doing five other things while I’m doing post-production. So, I’m not going to f*** it up by making the wrong decision in casting. So you’re going to read, or you’re not going to get the part.”
While Cameron did not make any wrong casting decisions in the film. Thinking of someone else in the role of Jack Dawson just seems absolutely wrong. However, their first meeting was far from ideal, which made the filmmaker wonder if the actor was the right choice for the role.

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Leonardo DiCaprio and James Cameron’s First Meeting

While DiCaprio ended up bagging the role over Matthew McConaughey, the first meeting between the two was quite dreadful. In a separate interview, the Terminator filmmaker acknowledged that he was unsure if The Shutter Island actor was the right for the role, but his ability to charm the people was a key element of the character which made James Cameron believe that the actor was a good choice for the role. 
James Cameron
“The only thing I’d ever seen him in was What’s Eating Gilbert Grape. He wasn’t that hot yet; Romeo & Juliet was still in production. He was just this charming guy with the ability to walk in and win a room–which was one of the fundamental things about the character, Jack. So then I had to assess how committed he would be and how his acting was.”
The filmmaker continued that during the audition process, he asked him to read with Kate. Instead, he chose to smoke a cigarette which upset James Cameron, however, he charmed Cameron with his performance making him believe that he was the guy for the role.
“I asked him to read with Kate. He did something that just rubbed me the wrong way: he sat there smoking a cigarette, slouching, as if the whole thing was too much trouble. I didn’t think he was paying attention. Then he gets up there and he does the scene. And it was like, boom, I saw it. He’s the guy.”
A still from Titanic
James Cameron continued that DiCaprio ended up rejecting the role because the role of Jack Dawson seemed too clean to him, and he wanted to add a little depth to the character by making him a drug addict, and he wondered how he can make the character great.
“Then Leo decided he didn’t want to do it. It wasn’t quirky enough for him. He wanted, I don’t know, warts or a hump or a cocaine addiction. I said, No, that’s not the guy. He’s like a Jimmy Stewart character, pure of heart. Then, there was a moment the lightbulb went on for Leo, and he realized that that would be a really hard thing to make great.”
While Leonardo DiCaprio was unsure if he could make the character look great without any bad habits, his portrayal made Jack Dawson one of the most iconic characters in movie history. As Dawson was a charming guy, selfless and compassionate and his chemistry with Rose DeWitt Bukater made the audience fall in love with the character.