Maturing dogs will change personalities. If it doesn't show up at 6-9 months, it'll show up at 12-18 months. IMO, I've found that most dogs' temperaments do not settle until after 18-24 months.

I forget Chopper's age, but it's highly likely that he's going through another fear period. Just treat him as you would if he was a puppy. Exude calm confidence, ignore his fears, don't push him past his threshold, and counter condition the triggers (in this case, strange people) so that he associates strange folks with positive experiences. Hint: if strangers want to pet Chopper, give them a treat and tell them to toss the treat back to Chopper. That way, he doesn't have to approach them, but he still learns that strange folks = good!