Stiller has frequently come of as narcissistic onscreen - he either revels in it and plays it broadly, as in Dodgeball or Zoolander, or plays the nice-guy nerd who is most clearly the only sane person onscreen. In real life, Stiller is a smallish guy who works out like a fiend - his physique, occasionally glimpsed on film, is pretty well sculpted, and it's like he can't resist winking to the audience sometimes as if to say, "Look, I'm playing a nerd, but we all know I'm not, right? I'm more awesome than that."
And that's fair enough...but it's not Walter Mitty. The original story ends with Mitty imagining himself facing a firing squad...I won't tell you how this one ends, but as you can probably guess by now, it ain't that.
If the movie were retitled Ben Stiller Is Secretly Way More Awesome Than You, I'd probably have less of an issue. Mitty's not even the most interesting character in the film - Penn's action photographer would be a much more compelling guy to globetrot with, and the actor seems at his most relaxed in ages inhabiting the role. Wiig is sweetly nice, and a refreshingly real, somewhat age-appropriate beauty for Mitty to woo. And Patton Oswalt's customer service representative for a dating site could easily have his own movie, and I'd watch it - after Young Adult, I know he can make that kind of mundaneness fascinating. Hell, he'd have been the perfect Mitty, really - what was Big Fan if not a rehearsal for something more like this?
By my count, Stiller has now directed exactly one movie I've liked without reservation - Tropic Thunder, in which he actually let his costars steal the show. The rest - The Cable Guy, Reality Bites, Zoolander et al - tend to start off with great ideas and blow it by the end. Ironically, that's more like something the actual Walter Mitty would do than anything in this movie.