I'm guessing you mean a custom NPC? To speak you need to scan them, and choose the talk option. However if you haven't given him a speechtree then I don't think it will work. As for the script not working/loading there is/was a bug posted in the bugbase regarding this. I don't know if its been fixed, or if it has been, whether it is retroactively implemented for customs already present or just for new ones.