The weight of the soul is a fitting title reference for Alejandro Gonzalez Inarritu's devastating English-language debut, 21 Grams, in which the lives of a former-druggie mom, a dying math professor, and a religious ex-con become entwined after a fatal car accident. Naomi Watts, Sean Penn, and Benicio Del Toro ignite Guillermo Arriaga's intense script with the fiercest portrayals of their careers. (One wonders whether Penn managed to smile during the six months he turned in both this and his harrowing, Oscar-winning ''Mystic River'' performance.) Iñárritu (''Amores Perros'') uses indirect detail -- a rising flock of birds, a mention of a child's shoelace -- and a non-chronological structure to create a crescendo of feeling in his bold addition to the genre of multiplot dramas like ''Magnolia'' and ''Traffic.'' EXTRAS Sadly, none; on such an emotionally and organizationally complex film, cast and director commentary would have been enlightening.