Sylvester Stallone "just wrote about what (he) knew" when making 'Rocky'.
The 77-year-old star penned the script for the 1976 film – which spawned the iconic franchise featuring Stallone as Philadelphia boxer Rocky Balboa – and used his own life as inspiration during his initial ideas for the character.
Speaking at Toronto International Film Festival, Stallone said: "I just wrote about what I knew. I was writing a lot about this little kind of mentally challenged guy who happened to have a lot of heart.
"I wanted to write a movie about a guy who says, 'I'm not great at all, never will be. I fight great fighters. But I just want the opportunity to go the distance.'"
Sly penned the screenplay for 'Rocky' in just three days but was disappointed that Hollywood executives showed little interest in the project at first.
He said: "Nobody wanted to make it. It was my best writing too."
Stallone is known for his action roles in franchises such as 'Rambo' and 'Rocky' and is aware that he did not have the ability to be a "Shakespearean actor".
He explained: "I didn't have the bones to be a Shakespearean actor. It's important as an artist to know what your strengths are, but more important to know your weaknesses."
The 'Cliffhanger' actor previously admitted to being in a three-day "fury" when he collaborated with producers Irwin Winkler and Robert Chartoff to get 'Rocky' on the big screen.
Stallone recalled to GQ magazine: "I had this opportunity with Chartoff and Winkler, and I wasn't going to let it slip by.
"I was young, and I had an incredible amount of energy, and I wrote it in a fury. I was very excited about the whole thing. I had a feel for the streets, and I loved films like 'Mean Streets', and 'Marty', and 'On The Waterfront', and I felt inspired."