The film centres on Ned's relationship with his three starkly different sisters (played by Emily Mortimer, Elizabeth Banks and Zooey Deschanel) and shows the hippie farmer as he faces the consequences of his idealistic lifestyle.
The actor told MTV News: "I immediately go to jail for selling marijuana to a uniformed police officer.
"However, the character is not dumb. The character just kind of appeals to everybody's higher angels and thinks that if he throws goodness out there, it will in turn come back."
Rudd, who stopped cutting his hair and grew a full beard for the role, said that he would phase the new facial hair out in stages.
"I'm going to shave tomorrow. I'll do it in segments. You can never just shave a beard off. You have to do it piece by piece," he explained.
"Usually, the first thing I'll go for is the James A Garfield or the Zachary Taylor. Just the chin shave. Actually, I don't know if Garfield had that. I'll have to Google it."
My Idiot Brother will premiere at the Sundance Film Festival later this month.