The director initially enthused about drafting Willis for his upcoming cop movie A Couple of Dicks, although the pair sparked speculation last month of a rift over Willis's demands to add his own creative input.
The Die Hard star later brushed off the reports, though Smith has now insisted that he did struggle to direct him on set.
Contactmusic quotes the helmer as saying: "Bruce Willis is probably the most intimidating actor I've ever worked with. I find him - and I don't mean this in an insulting way - undirectable.
"He has all sorts of range, but what he's predominantly known for is playing the guy who holds his gun down by his side and saves people. How do you tell Bruce Willis to be Bruce Willis?"
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