I imagine that by then they would have taken on extra staff like letterers.
You certainly can imagine, but I don't think it reflects the situation. Artists very often lettered their own comics - Dave "Watchmen" Gibbons still
letters all his own comics, having done them all by hand up until he had a digital font of his hand-writing created.
Such staff as Hergé had access to was really very small in number, maybe a couple of other artists and his colourist, and at the time he worked between his front room in Brussels and his house in the country, not in some pool of allied trades.
that someone else did letter pages, but not necessary, and it certainly seems highly unlikely, if not just plain impossible, that anyone in his small band would have tried to alter the dialogue without Hergé having signed off on it first.
but like I say the body language is at odds with the words
No it's not; Tintin is aware that the crashing and banging of the Captain's efforts will probably get a reaction from their captors, which it does.
He has gone to a lot of trouble to lull them into a false sense of security by being cooperative, and, as his plan is just
about to come to fruition, he's simply frustrated that the Captain would be about to blow their opportunity.
which would be a major slip for an artist like Hergé
It would be a much
bigger slip for him to have staff changing his dialogue willy-nilly...
an "I dare you" attitude with a touch of sarcasm just does not feel like Tintin.
...so we have to take it at face value that it is indeed Hergé writing, and that the dialogue and image are as he wanted.
It's funny that you get terribly worked up about changes made to the Methuen book which were extensively discussed by the translators and Hergé, and can't accept them as an expression of what Hergé wanted, but have to defend what you see as uncharacteristic in th eoriginal, by saying that it must result from it not being by Hergé.
Hergé was pretty much in control of the whole process, and there's absolutely nothing to suggest that his words were being changed by unknown third parties.