I've interview three times in the last seven years, with small, independent companies. In each, I talked with several developers, answered tech questions, wrote a little code, and talked with managers. They generally try to find out if you're legit, easy to communicate with, personable. They all expect 50+ hours/week commitment (or more!)
If you are interviewing with a large company, they will likely have a more elaborate and structured process. Perhaps a personality test, though I've never met anyone who's had to take one. Some companies require drug testing, but I think that's rare.
Learn details about the company/product beforehand. Dress smart, well-groomed. Be relaxed, confident and honest. Let your personality show. If they won't like the real you, you won't like working there anyway. Good luck!