Actually, if you look at the movies, except where they're trying to make a joke about how bad the legroom is, the characters almost inevitably are seated in a row with unrealistically long seat pitch - EVEN when the point is how cramped coach is.

I found the recent Karate Kid remake particularly funny for this, since the row they were shot in had unrealistic amounts of leg room and the other rows shot did not.

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As for the general question of the thread, basically, as others have said, bring it up with the FA immediately. I don't agree with those who are blaming it on the passenger-of-size, and IMO these situations SHOULD be accommodated under the ADA - but not at the expense of the adjoining passenger. The POS has got to travel, and are undoubtedly as uncomfortable as you are. The problem should be neither yours, nor theirs, but rather the airline's. The basic responses that PTravel outlined are pretty good.

For those with status (or where airlines allow paying for them), in some cases getting an exit row seat will help with this, as the armrests in many cases (not all, see your favorite seatmap/seat-reviewing site to check) on those will be fixed.

Note also that it isn't always fat people who have passenger-of-size problems - the worst seatmate I've had was a very tall, very broad-shouldered guy who fit just fine at seat level and took up an extra half seat at shoulder level. Similarly, somehow when the related issue of recline comes up a lot of the very tall people are asking for a right not to be reclined into and get a bit more sympathy.