Peter Sellers had a relatively unique, old-school way of finding his characters’ essence — in fact, Joseph Gordon-Levitt’s technique is similar to his. As Sellers would tell Playboy in 1962 (via Far Out):
“I find out how the character sounds. It’s through the way he speaks that I find out the rest about him. […] After the voice comes the looks of the man. I do a lot of drawings of the character I play. […] After that I establish how the character walks. […] And then, suddenly, something strange happens. The person takes over. The man you play begins to exist.”
Sellers’ long, varied career is a testament to his acting method, but the process of finding a character occasionally raised some red flags. When it came time to discover Chance, Sellers noticed a big discrepancy between the character’s physique and his own, he told Don Lane. Since Chance worked a “sedentary” job, he was relatively heavyset. In contrast, Sellers was quite thin.
Enter Robert De Niro. The actor had recently put on around 60 pounds for “Raging Bull,” which inspired Sellers to gain weight for “Being There.” The comedian spoke about the influence while chatting with Lane:
“I noticed that Robert De Niro just put  pounds on for a movie, with the supervision of a doctor, and I agree with that, because padding never looks right. I mean, you pad your body out and then your face doesn’t look right.”
Chance’s weight was a relatively minor trait and Sellers could’ve simply made the character thinner if he so desired. However, the actor’s willingness to alter his body in order to do Chance justice displays the level of commitment to his art.