I was first invited to an informal meeting over coffee with the CEO. I was unable to make it at the time. But, my referral being strong enough, I was directly invited to meet the client, due to time constraint. Following the client's (Director of Planning, Design & Construction at a Hospital) request in hiring me as an employee extension to his team, I was made an offer.
Normally, the first informal interview (at a coffee shop or restaurant) will be to get to know you, as well as to provide a well-rounded insight about the company.