One of Charles Williams most important contributions as a theologian seems to have been to clarify, explain and emphasize the idea of co-inherence - and its related ideas of exchange and substitution.
The concept is that we are members one of another, and with Christ, and therefore can substitute for one another, for example in the exchange of suffering.
As when I agree to take on your anxieties, and your anxieties might be taken on by me - or by a third party; so that we 'bear one anothers' burdens'.
This also links to the taunt of Christ on the Cross - He saved others himself he cannot save" - this is assumed to be a general observation about mankind - to be saved we must save others, and be saved by others.
This makes the main plot of Descent into Hell, and features in the mature theology of He Came Down From Heaven and Descent of the Dove.
But Williams pushed this idea too far, in my opinion.
In Descent into Hell, especially, he detached it from Christian life, and made it a kind of technology - almost a magical therapeutic practice for the alleviation of anxiety and pain.
Williams biographer - Alice Mary Hadfield, says she had a long running disagreement with Williams on precisely this point - the extent to which exchange and substitution could be practiced apart from Christianity: she said it couldn't - he said it could.
This strikes me as a residue of C.W's fascination with ritual magic as a young man, and his decade long involvement with A.E Waite and the 'Golden Dawn' movement.
Perhaps it was things like this which made CS Lewis say in a letter to his brother, tongue in cheek, that for all his sanctity (which Lewis acknowledged and appreciated more than almost anybody) there was something 'combustible' about Williams - the sense that in another era he would have been burned for heresy, and (in a sense...) deserve it!