This turned out to be quite good. Willis plays a washed-up, alcoholic, exhausted Detective, who is asked to escort Mos Def down the street to the court house to testify. It's just past 8am, and he has to be there before court closes at 10am. So, nicely real-timey, then. Turns out that Def's supposed to testify against a bucket-load of Willis's collegues, including David Morse, and (as we know he would) Willis does the surprising, unexpected, Right Thing, and so find himself chased by his own force, protecting a criminal.
Willis's character starts off out of breath, and pretty much stays that way - this doesn't suddenly give him superhuman strength and allow him to go eighteen rounds with some huge bruiser. There's a nice sense of despair about the whole thing. Def's far too nice, but since he's firmly in the Good Guy role here (despite being a criminal), I suppose we'll let that go. I even managed to put up with his naselly voice throughout.
Morse made a great Villain, just creepy enough, and Willis always does a good job. Could have done without Willis's mustache, though.