The final five words of your question define what is going on here. In my view this is more than an interview, you are being invited to their company so they have the opportunity to see whether you will 'fit' in their organisation. And from a small business perspective, potential negative repercussions abound if they get this wrong. This is more pertinent in a small organisation where interpersonal dynamics hold greater potential of influencing group, departmental and organisational behaviours and outcomes.
Best outcome for these guys is to find someone who can do the job well in a professional and competent manner and whose personality, attitudes and behaviours are aligned with their individual/group and organisational ethos/ideology/game plan. Get it right, you will all be happy, get it wrong and the negative repercussions could be dire and go way beyond the individual. I am happy to provide links to research by organisational psych’s and others who study the complexities of the interaction between individuals and organisational structures/entities.
With respect to interview -v- discussion etc. although I agree in principle with much of what DJClayworth says above, my take on it is a tad different. I hope this provides an alternate way of looking at the situation.
The interview process generally involves a dynamic in which information sought and obtained is unidirectional and controlled by the more influential individual or group. This is set up so that they can engage in a well-informed and structured decision making process. The dominant features of this process are the rationality and objectivity, which underpin the elements of such an interactions. Seems they got that stuff sorted from your Application and CV.
In the moment they invited you to have a discussion with them the power differential shifted. The dynamic involved in conversing successfully requires that all parties actively listen and hear the view expressed by the other and respond in a considered and hopefully insightful manner. This is a process in which you will have to engage if you get the job. It is a process that relies on a natural connection and ability to communicate with relative ease whilst respecting the different individual roles and responsibilities in the organisation. This is not something that can be up-skilled later on, it's not about the individual its about what happens when a group of like minded people get together to work on a project and how that evolves.
Do you see the difference?? Do you understand why these people may want to 'get' who you are, what your underlying moral characteristics and behaviours are like and whether they can imagine working closely with you on various projects over a long per8od of time. You might also like to consider these things as you are chatting with them. In your mind, appraise whether they as individuals and their company ethos fits with you and how you want your work life to evolve. Remember, we spend more time at work and we attribute much of our sense of wellbeing to our successes (or not) in our professional life. Many define who they are by linking their sense of self to their occupation.
I hope the above makes some sense to you and that it provides some clarity for you - even if it comes from an 'out there' perspective.