What's the job? Is this the only interview round? There are a few variables, but you may not get a skills or aptitude test, or some other final pre-screening measure, under the informal label. The rest is highly dependent on company size, how they handle HR and other runners in the process.
I was in informal 'chats' that lasted several hours with a full panel and covered everything about my past work history, the role and the company; likewise I had 'interviews' where I had the job the moment I walked through the door, and it was mostly a case of making sure I liked and was liked by the people I would be working with.
At minimum: know yourself (your CV and job application!); know the company (they have a website/social media presence for a reason); stay calm. If you find yourself in a position where the interviewer is selling the job to you/grand tour - be positive and take active interest. Don't forget that you will be finding out about the business at the same time as they'll be finding out about you - prepare some questions and add some on the day if you see/hear something you like or don't like.
Good luck.
