It's easy to think 3D is 3D, and the only difference between Avatar and Clash of the Titans is the difference between corny and extra corny. But there are actually two ways to do it -- the real way and the easy way. The real way is to actually shoot it with a two-eyed camera that films things the same way your two eyes judge depth and form a 3D image from it.
This camera is fancy and therefore expensive, so there's a lazy way too, which is to just shoot the whole thing with a regular one-eyed camera like a regular movie, and then you have some low-paid overseas workers basically cut out the actors digitally and turn it into a digital diorama.
Just like this, but on the computer.
That's why a lot of people complained about 3D movies like Clash of the Titans and the 3D parts of Superman Returns. They paid $13-$20 for a ticket and got something that looked like that dinosaur UFO shoebox there. Sure, you can convert 2D movies to 3D better than that, but it costs a lot more time and money, words that make many studio heads cough violently.
Fun Activity: Cut out the characters in this Clash of the Titans screenshot. Congratulations, you just saved yourself $5.