I think we have all seen the adverts for this film and they paint a pretty clear picture of the plot - a man who is unhappy with his life, imagines ways in which he could handle situations better, he is then given an opportunity to change his ways and on this adventure becomes a 'better man'. This is the story ladies and gentlemen, and there's not really much more to it than that - unfortunately, plot wise, I feel kind of let down.
But, despite this, I think of this film as a great - I believe Ben Stiller wrote, directed and acted in it as the main character and his comedy genius really shows through. The imaginings of Walter Mitty, are shown so well - at times you believe it's actually what's going on at the time, until we are dragged back to the real world along with Mitty. The adventures that Mitty goes on, can be a bit unbelievable and coincidental, but are fun and actually enjoyable - the clues that Mitty follows on his hunt for a mythical photographer are well put together and you find yourself completely engrossed.
Actors in this film, apart from Ben Stiller, are not that well known to me personally - The mythical photographer that Mitty looks for is played by Sean Penn and, although I remember him from his role as Mickey Cohen in 'Gangster Squad', nothing else of his rings any bells. The girl Mitty day dreams about, Kristen Wiig, I seem to remember as a voice actor from 'Despicable Me', 'Despicable Me 2' and 'How to Train Your Dragon' but again nothing else where she is actually on screen.
Overall, a thoroughly enjoyable film if you're not too bothered about a complicated storyline - some hilarious laughs (I reference here, Mitty's reference to Benjamin Button) and just something for the family to watch - go out and watch it, it's worth it