I am currently undergoing this process as well. The OCL appointed a clinical investigator whom has observed me interact with my child, interviewed me, as well as interviewed all those people he has deemed as being influential in his life. This includes my girlfriend, and baby-sitter, etc. My assumption is that our son's Mother has been through it too.
The Investigator didn't seem too interested in documentation - as he is to write a report stating his opinion as to what would be best for our son - rather than the whole he said / she said aspect of it. I have found the experience to be not un-welcome, as the investigator has listened well to me. I am hopefull that he will see things as I do, and that I am not so biased in emotion to have a skewed perception of what is really going on.
I beleive there are other instances when the OCL may appont the child a lawyer, but as our son is so young perhaps it was not the bst solutuion in our particular circumstance.
My best advice to you is to conduct yourself in the most appropriate manner you are capable of in all things concerning your children. Be yourself in any interviews, and trust that whomever is assigned sees that you are an indespensable part of your childrens life.