What's with the dad yelling at the bottom of the stairs at Johns mum?
What is he making her go down there for? and why does she put up with his crap?
I think he was too drunk to get it/himself up.
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It's not "sci-fi", it's SF!
He was asserting his working class male authority. It's not so much a case of he can't walk up the stairs as why should he? I thought the scene worked really well in suggesting the abusive nature of the relationship between John's mother and father, and also in offering clues at to the origins of John's own personality issues.
"That, - Captain Bligh, - that is the thing; - I am in hell, sir - I am in hell."