I think Alexa Chung may be the only person in the world able to wear overalls and not look silly. I mean, she still does look a bit silly, but it's Alexa-silly, and it suits her, which kind of makes it okay.
I'm not suggesting you run out and get a pair of overalls anytime soon. Far from it. In any case, you'll have a hard time finding some. I recently went as Bob the Builder to a "B" fancy dress party and searched for a pair, ending up with some $15 ones from Vinnies. Can't really go wrong with $15 overall shorts for a one-time wear!
Me as Bob, with my friend Kristy who went as blue. This is as far as my overall shorts wearing will ever go.