How do you answer 'When interacting with clients and co-workers is it better to build friendly relationships or is it preferable to maintain strict professionalism' in a job interview?

To your question I would say to build friendly relationships with your clients and co-workers. I have worked in places where people weren't so friendly and I didn't work as efficiently as I should have or would have if I worked in a friendlier surrounding. I now work in a place where I am friends with my boss and I get the things I need done right and on time. Just make sures it's not too friendly. You don't want them to feel as if they can do WHATEVER they want, but that they know they have limits but still are free.