In his documentaries, Errol Morris has helped free an innocent man and made oddball small-town life seem appealing. But that's almost nothing compared to the feat he achieves in "Fast, Cheap & Out of Control."

Only a filmmaker of Morris' considerable skill could take the threads from four seemingly unrelated interviews (with four very different men) and weave them into a coherent narrative, as he does with this funny, thought-provoking and sometimes touching movie.

His subjects this time around are Dave Hoover, an animal trainer with the Clyde Beatty-Cole Brothers Circus; George Mendonca, a devoted topiary gardener; Ray Mendez, a specialist on the behavior of African naked mole-rats; and Rodney Brooks, a robotics expert at MIT.

Interviews with any one of the four men would have made an interesting film on their own, but that would have been too easy for Morris, who instead lets them share their sometimes weird but often intriguing theories on the nature of existence — even though they same to be unaware that they're doing so.

For example, Brooks and his MIT colleagues have invented complex, insect-like robots that move of their own accord. Describing his creations with a Frankenstein-like fervor, he envisions a future in which robotic lifeforms have replaced human beings as the dominant species on the planet.

In a similar vein, Mendez explains the behavior of mole-rats, tiny, hairless, subterranean rodents whose social structure is more akin to insects than it is to that of other mammals.

And on a less futuristic note, Hoover and Mendonca lovingly look at their trades, which are dying due to lack of interest.

Deftly mixing black humor and drama, "Fast, Cheap & Out of Control" ultimately succeeds because it never fails to be fascinating. Though his subjects may come off as kooks at times, Morris refuses to poke fun at them. Instead, he lets them be all too human.

He also shoots things in his trademark conversational style and with his characteristic use of different formats, including shooting in black and white instead of color, using videotape and 16mm and 35mm film stock, and employing old movie footage.

"Fast, Cheap & Out of Control" is rated PG for violence, much of it animal violence, and some brief partial nudity (in particular, some skimpy outfits worn by Hoover's fellow circus performers).