Daniel Radcliffe, perfect as Harry in the first film and good in the second, falls just a bit short of the depth needed for "Azkaban." It's certainly not for lack of trying; he's a good young actor, and he's not too bad. It'll be interesting to see how he does in "Goblet of Fire" as he gets older. Michael Gambon is, of course, no Richard Harris. Dumbledore exuded magic in the first two films, and unfair as the comparison may be to the man stepping into the role following Harris's death, Gambon seems flat.
As much as the early part of the movie seems to drag, the rest, once we and Harry figure out what's going on and what must be done about it, moves smoothly and briskly, and the action is well executed.