I'm watching 1966 now, and apparently there's more for an actor to do with the part of the "nice guy" than I'd have thought. When Joe's suffering, Joel can do some really good acting. Once Barnabas arrives, not just Joe but most of the established characters have their personalities and life-situations "neutralized". They become generic bland protagonists, who have to stand in the background wondering what's going on, while the new characters handle the real business. I don't think any actor can make that interesting.
Forbes Part One was a nice guy too, so the transition was easy and gradual. Later Forbes and 1968/9 Joe let Joel do a lot more of course.