This might be exclusive to the Swedish dubbed version, but in Astérix And The Big Fight, I was kind of annoyed with how the soothsayers' voice was. It sounded too deep and dark and emphasizing and extravagant and... showy. And at about this point, I realized — of course. He's a con man. He isn't supposed to sound like an actual person, he's supposed to sound like the idea of a fortuneteller, the stereotype — and truly, he does have a slightly different voice the few times he mutters to himself, and entirely different when he goes insane toward the end. Also, Infirmofpurpus' voice sounded so... off, always speaking in a slightly awkward fashion, but what really bothered me was that his voice seemed to be cracking, or at least hoarse. Then I understood that... well, he was terrified out of his mind, so when he screamt he had to be screaming as loudly as he could. And living next to the crazy Gauls, he'd have to do this quite a lot. Of course his throat would go sore and his voice would go hoarse after a while. - Farmelle
This is also present in the original French version. The soothsayer is hammy as hell (and deep) when he's pretending, but as soon as he gets nervous (like each time he's talking to the centurion), his voice gets higher and he even stutters at times.
In Asterix and Obelix All at Sea it's shown that drinking the magic potion while under the effects of it will turn you to stone, as it happened to Obelix when he finally succeeded in drinking it, and it's not clear if Obelix returning to flesh but as a child was the effect of Getafix' attempts at curing him (because, being of stone, it's at least doubtful Obelix could actually drink the healing potions) or just what naturally happens. If it's what naturally happens... Roman admiral Crustacius became a statue after drinking a barrel of potion while under the effects of it already, and Caesar placed that statue in the circus, hoping that one day the lions will eat stone or Crustacius will return to normal.