I don't know myself but from what I've been told. It's Sporadic day dreaming but far more complex than regular day dreams it's like entering another world. The people and the world that are created by the mind are incredibly complex and realistic. People that have walter mitty syndrome also write themselves into a script that orginated from a preexisting show. But what really separates it from day dreaming is that all the character have names and there are often complex plots involving these characters. Often people with this disorder set aside time to revisit and indulge in this world. I thought I was the only person with these fantasies until I went to a counselor. But even after seeing a counselor I still had a lot of questions unanswered. My counselor agreed that my case was quite unusual. Most suffers have boring lives and there fantasy involve personal triumphs. Neither apply to my case. Take a look at what this walter mitty sufferer had to say:
my daydreams are fairly typical of walter mitty from what i've heard i tend to take inspiration from books, films, cartoons anything really and just change the story so that it involves me, these stories can range from something as simple as what it would be like if i thought my dog a new trick to the completely bizzare like having awesome powers and fighting gods and hoards of demons kick ass matrix style :-p
they also tend to be quite topical for example now that the holidays are coming up i create scenarios for thngs that could happen during the summer and what i could do, i find it quite useful because i'm generally well prepared for most scenarios that present themselves however even though i'm usually quite certain what action will provoke which response i can tend to make somewhat spontaneous decisions because with out spontanuity wheres the fun :-p