in the first film - the snake never really spoke.
No, i think Ron will where a wig when they try to break into gringotts seeing as they didn't have enough polyjuice but in the ministry, i assume they will bring in other actors but i think with Godric's Hollow - either they will scrap the polyjuice potion or again bring in other actors. Its the only way to do it but i think for godric's hollow it would make sense if they just scrapped the potion.
I am re-reading HBP and am wondering why DD always used different people to give the scrolls with the date and time of their lesson on? Why not just use different owls, or something? Is DD actually in the castle when he gives it to these people or do they get it by owl?