How Does Casting Work When it Comes to Finding Specific Body Types?

Often in movies there is a character that is mocked for being unattractive in some way. Obviously these things are hard to fake so they just cast real people with the physical qualifications. How do they go about finding people to appear in a movie?

Ashlie Atkinson: 

There is a lot about the way this question has been asked that gives me pause (and frankly, some of the answers sent my eyebrows skyward as well). I will address this from my own personal experiences as one of the "fat and uglies."
 
I live in New York, not Hollywood, and make my living as a professional actress. I get auditions through my agent and my manager, just like the ingenues. Roles I audition for, and book, are probably 50% look-specific and 50% not. But I am, as is necessary in my business, fully aware of how I look. My fatness is not a secret that is being kept from me, that I feel uncomfortable with people addressing. I am fat. I also feel very attractive in my everyday life, because at the end of the day, what I do at work is PLAY A ROLE. TV and film are not life. I do not get my self-esteem from the outward attractiveness of the parts I play.
 
I view playing a role that is described as "fat" or "unattractive" as a challenge and an opportunity. There is a widening chasm between how people on TV look and the appearances of the people watching at home, and I am proud to be an actress that women "of a certain size" can identify with. To me, it's not about being a freak. It's about portraying characters that could live in the real world, that are mothers or stage managers or fast food restaurant employees, that go through life without the benefit of plastic surgery and personal trainers. I am not an actor so I can look pretty, or to collect a paycheck and be laughed at. I am an actor because there is nothing on Earth I enjoy as much as acting. 

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