“The Secret Life of Walter Mitty,” directed by and starring Ben Stiller, is a very positive and uplifting picture, something that’s rare in this age of bleak and pessimistic cinema. It’s a movie about seizing life by the throat and taking risks, making spontaneous decisions and going on wild adventures.

It’s a cross between sprawling-adventure picture and quirky, deadpan comedy. It has some sweet and funny moments and fantastic visuals, but unfortunately, the story and characters aren’t all there, and it goes for cheap, forced sentimentality.

Based on the short story by James Thurber, the movie revolves around Walter Mitty (Stiller), an ordinary man living an unexciting life. He works at Life magazine, handling photo negatives, and most of his coworkers don’t seem to know he exists. As a result, he has a wild imagination, causing him to space out and transport himself into fantasy (telling off his jerk boss, or suddenly jumping off a subway platform into a building, saving people before a bomb explodes).

This is easily the worst and most annoying aspect of the entire movie. Walter lacks confidence and dreams of a more adventurous life, but Stiller can get that point across by showing us one of these fantasies or maybe two. But, by about the eighth dream sequence in a span of about 15 or 20 minutes (I’m exaggerating, but only a little bit), it gets stale, and as a result, the beginning of the movie is a slog to get through.

When we aren’t subjected to the fantasy scenes, we watch Walter timidly stumble and stutter through his sentences, talking to others or walking around looking sad.

Things do pick up a bit: A photo reel from famous Life photographer Sean O’Connell (an intensely funny Sean Penn) comes in, and Walter discovers that one is missing (which is going to be used as the cover photo for the very last published issue). In hopes of adding some excitement to his life and also to impress his crush, Cheryl (Kristen Wiig), Walter decides to track down the missing photo. The journey takes him to Greenland, Iceland and the Himalayas.

The photography in these scenes (by Stuart Dryburgh), showing the vast, mountainous landscapes is the only really breathtaking thing about the film, though the fantasy sequences also look very stunning.

The other thing I liked about the movie was the supporting cast. Whether it’s Penn, Wiig, the always-reliable Adam Scott as Walter’s jerk boss, Patton Oswald as an eHarmony employee or Shirley MacLaine as Walter’s mother, the entire supporting cast is spot-on, providing humor and partly diverting our attention from Walter’s blandness.

Yes, Walter Mitty is a bland character, played by Stiller in the same uptight, slightly timid and mopey manner of practically every other character he’s played in the past. At this point, it’s old and not very funny anymore. He’s utterly shallow and goes through the exact character arc you expect him to.

In fact, for all its sense of adventure and wonderment, the movie doesn’t feel very adventurous, but stilted and bland; the rousing adventure scenes don’t always gel with the mundane, deadpan back-and-forths .

The film is also mostly devoid of surprise, and the few surprises there are (toward the end) really aren’t all that surprising but instead sappy and frustrating.

On top of that, much like “Forrest Gump,” the picture’s whimsy tends to get infuriating at times, all before it builds to its predictable and underwhelming conclusion.

Stephen Frears’ “Philomena,” with Steve Coogan and Judi Dench, is a perfect example of a positive and uplifting movie that came out this year but one that doesn’t resort to cheap sentimentality and develops interesting characters. “Walter Mitty” isn’t terrible, but considering Thurber wrote the original story in 1939, it’s not surprising that it feels so outdated and insipid.

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