No interview scenario is ever the same.  It is critical to quickly assess the environment as soon as you walk into the room.  First impressions are important.  Some interviews will be casual; others will be formal.  Nonetheless, establishing rapport is critical.  You are highly unlikely to secure a role based on technical skills alone.

Once you've sat down, interviews will often commence along the lines: tell me about yourself, what do you know about the company or why do you want to work here?  You should be able to address these questions with ease in a concise, well-prepared manner.

Despite all this information, it is important not to overthink the interview and to try to enjoy it.  Every interview is a two-way interview and both parties should walk away satisfied.  As a rule of thumb, an even talking split is generally a good sign.  Listening to the questions carefully will allow you to deliver the best possible response. 

Prepare questions that demonstrate your research and intelligence e.g. don't simply ask who are their main competitors - dig a little deeper into how they differentiate themselves.  You should be able to drop in your questions throughout the interview.  At the very least, you should be able to ask them at the end of the interview.  At the end of the interview, if you're asked if you have any questions, you should!  It is a great way to cap off a strong interview.