A big 6 editor once told me that my street-kid Lee sounded way too educated. In reality, she said, he'd be at best poorly-educated with a limited vocabulary and wouldn't necessarily have the words or emotional maturity to deal with situations the way I'd written. She had a point. When I stepped back, I realised that Lee was speaking with my voice and while I'd tried to dumb his dialogue down a bit, it clearly wasn't working. It took me a long time to get inside his head and put his own words into his mouth rather than mine. And once you step inside somebody else's skin, even in fiction, you become that person.
As a reader or a writer, to get that emotional connection requires some effort on your own part. It makes you vulnerable when you walk in someone else's shoes, feel what they feel - the good and the bad. It can make for an amazing experience. But you leave something of yourself behind every time.