He didn't really decline the role, but the part was recasted
Samuel L. Jackson as Alonzo Harris
Training Day


Samuel L. Jackson as Alonzo Harris
Training Day
Did You Know…Samuel L Jackson Was Meant To Be The Lead In Training Day?
We all know Denzel Washington did a brilliant job in Training Day, which even lead to his first lead Oscar win, but can you imagine anyone else walking in detective Alonzo Harris’ shoes?
Well originally director Davis Guggenheim was all set to direct the film with Samuel L Jackson and Matt Damon in the two main roles. However, Denzel Washington signed on to the film and when this happened Guggenheim requested to be replaced (I guess he really wanted to direct Samuel L Jackson) This lead to Antoine Fuqua taking on the director role and the rest as they say is history.
So would the film have had the same impact, I think probably not, cause prior to this we had seen Samuel L Jackson play the baddie role. However this was a first For Denzel and I think that was a huge reason why the film did so well.