Except that's not quite what happens. Grandpa Jo is the one out of the four saying he wants to get his strength back to help Charlie out with his jobs so he has time to play. He might not just be talking about physical strength either it could be mental. Because he seems pretty miserable but as soon as Charlie is back he is excited and has a bit more energy, stopping his complaining so that Charlie doesn't see. He offers to give up tobacco and try's to turn money down from Charlie to buy some when he sees Charlie buying them food. He then spends the money that is supposed to be for tobacco on a chocolate bar for Charlie so that he might be able to go to the chocolate factory and experience the stories they have heard about it. When Charlie doesn't get one he tells him that it's okay he will find a golden ticket, so not to give up. He is the only person that is saying that to Charlie everybody else thinks it's unlikely he will get one. But not grandpa Joe he believes in him and wants Charlie to have high hopes not just for the ticket but life. When Charlie does eventually get the ticket Grandpa joe doesn't just jump out of bed shouting "go on Charlie lad let's get crackin" and skips off to the factory. He only attempts to get out of bed when Charlie wishes he could go. Because he doesn't want to let him down and suddenly has something to look forward to. Like I said it might not just be a physical disability for him being in bed but mental.

So I suppose there are multiple ways to look at it, you can look at it like Joe was lazy and then jumps out of bed as soon as he hears that he can go to the factory. Or you could look at it like Joe is trying to give Charlie something to look up to and get out of poverty even though he doesn't believe in himself. And that when the opportunity to share an adventure at a place that they both have dreamed about comes along he is suddenly filled with joy and able to break any mental barriers. Doesn't matter at the end of they day though because it's not real haha.