The 51-year-old star began writing his own songs when he was just 12 years old and when his older relative found his "rap book", rather than reprimand him for his use of profanity, she simply wrote a note in the journal urging him to be more "intelligent" in his choice of words.
Appearing on 'Late Night with Seth Meyers', the host said: "One thing you did that was fearless but maybe didn't look that way to people, I first knew you as a hip-hop artist and at a time when this was not what a lot of people were doing, your lyrics were very clean. That choice was a bigger choice than I think it probably looked to people."
Will explained: "I was 12 years old and I started rapping. So, I had my rap book, and I was writing my stuff and I had all my little curse words in my rap book. And my grandmother found my rap book.
"And she just turned - she never said nothing. Turned to the back page and she said, 'Dear Willard, truly intelligent people do not have to use words like this to express themselves. Please show the world that you're as smart as we think you are. Love, Gigi.' I read that, and I was like, 'F**k.'"
Will has recently returned to action movies in 'Bad Boys 3' and while he was initially excited to get stuck into the scenes, he soon found his body can no longer heal the way it used to so was keen to bring in a stunt performer.
He said: "When I started, I was like, 'Bad Boys 3, we're going right back. I'm doing my stunts. I'm going to get it. I don't need no stuntman, I'll do it. And three days in, I was like, 'Ay, where's my stuntman?'"
The father-of-three thinks he needs to look to his younger self for advice as he's o longer as "fearless" as he used to be.
Asked if he thinks about what he'd have advised his 25-year-old self, he said: "You know, I've thought about that. And I think I wouldn't give myself advice, I would probably ask myself for advice. When I was 25 years old, I was so ambitious, and I was so driven and I was fearless.
"And as I've gotten older. I've started getting a little bit more fearful, and I would ask myself, I was like, 'Why are you so crazy? Why do you think you can do so much?' There's such a power to naiveté. It's like once you know, you can't unknow."