it would never occur to me that someone in the dream extraction business would not be accustomed to improvising

I should clarify, it wouldn't occur to me unless there were more clues than 'likes to have a plan when about to embark on incredibly difficult mission to carry out believed-impossible objective' and 'doesn't like it when partner abandons plan to flirt with dead wife'.

There's no indication Arthur is unduly obsessed with planning, and when it all goes tits-up, he takes it calmly in stride and comes up with a brilliant and off-the-wall plan, with no indication he's doing anything unusual. Never has a man wandered through constantly shifting gravitation while beating up goons to tie his comrades into a lump and blow them up in a lift with greater sang-froid, and while that's charming, it's not exactly the sign of a man who's not comfortable with improvising.