Ooooh, I am confused now.
There are a number of possible ways of looking at it, I think:
If we take it at face value, then Steve is a genuine participant who is not 'in on it'. He is also an aspiring actor. Derren would surely have known this from his early investigations. In which case, he decided this guy's acting career was not significant enough to make people suspicious. This seems a bit unlikely to me.
Second possibility is that he is a genuine participant, not actually an actor, and the acting and marathon pages were fabricated to make us suspicious.
Scenario 3: Steve is in on the whole thing and is an aspiring actor. In this scenario, Derren has failed to do the adequate preparation and underestimated the likelihood that the audience would do their own detective work.
I really dislike scenarios 1 and 3 and desperately hope it will be #2 because that will spell a big twist for the audience. :)
I'm confused about the Wizard of Oz. I understand the references, but what will this mean? I don't know the film well enough to draw any conclusions from this.