Just as in the case of the comedies, these classifications are not intended to be written in stone.
You may prefer to put down "The Bonfire of the Vanities" as comedy (satire), or "The Siege" as action. Still, more of Willis' films classify as dramas than action movies - strangely, the public (and critic) eye prefers to see him as Action Man...
In any case, the "Drama" category contains Bruce Willis' best performances, some of them amazingly underrated and little known (notably "In Country").
Read about the "regular" Willis vehicles in the "Action" category, or explore some more surprises under the heading of "Comedy".