Here's some info on some of the characters they are still looking to cast for Constantine. I put the majority of it in the extended entry as it might be considered spoilery to some.

"Constantine" (Warner Brothers) - Casting Call

A film adaptation of the classic Vertigo comic book "Hellblazer".

"Constantine" will star Keanu Reeves as John Constantine, a world-traveling,
mage-like misfit who investigates supernatural mysteries, walking a thin
line between evil and good. He teams up with a female police detective
named Angela Dodson, who seeks Constantine's help while investigating the
suicide-like death of her twin sister Isabel.

The investigation takes Constantine and Angela through the world of demons
and angels that exists just beneath the landscape of contemporary Los
Angeles.

Production will begin on September 22, 2003.

CAST MEMBERS: Keanu Reeves, Rachel Weisz

PRODUCERS: Lauren Shuler Donner ("X-Men") and Richard Donner ("Lethal
Weapon")

DIRECTOR: Francis Lawrence

The film's casting directors are looking to fill a wide variety of principal
and background roles.

MALE ROLES FOR THIS FILM:

[Chaz], Caucasian, 20s - 30s. A tough, shadowy L.A. cab driver and one of
John Constantine's oldest friends. Chaz has some ties to illegal activitiy,
but is making an effort to stay out of crime. Even though his wife does not
like Constantine, Chaz and Constantine are very close. They would do almost
anything for one another in a crisis. PRINCIPAL ROLE
(Hmmm, maybe Shia LeBouf isn't cast after all? it seems he'd fit better in the role right below--krix)

[Beeman], Caucasian, 20s. A highly intelligent, offbeat young guy who
assists Constantine with special information and occult artifacts. He's a
bit skinny and unimposing, but makes up for it with his mind. Beeman reads
to Constatine and Angela from an ancient scroll, which explains what
supernatural troubles they must save the world from. This character has
been described as a "Seth Green type". PRINCIPAL ROLE

[Morgue Security Guard], any ethnicity, 30s - 40s. An eccentric, reclusive
man who works at night. Nervous and high-strung.

[Balthazar], any ethnicity, 30s - 40s. One of the three emperors of Hell
who Constantine discovers is behind a series of crimes being committed
around the world. Balthazar is one of the main villains of the film.

[Criminal], Middle Eastern, 20s - 30s. He escapes from a Turkish prison.
Slight, muscular, lean, not scruffy. He possesses the relic of the spear.
He tries to kill Angela and gets in a fight with Constantine.

[Father Garret], Caucasian, 40s - 60s. Angela's highly spiritual and kind
priest. He is seen in confession with Angela and again at the theological
society where Angela asks for her twin sister Isabel to get a proper
Catholic burial. PRINCIPAL ROLE

[Busboy], any ethnicity, 20s - 30s. Hard-working, clean-cut young busboy at
a restaurant.

[Korean Man], 30s.

FEMALE ROLES FOR THIS FILM:

[Ellie], Caucasian, 20s - 30s. An alluring and seductive woman known as a
"Succubus", a female demon that seduces men while they sleep. She is the
only Succubus to ever mate with an angel. She is pretty and vulnerable yet
can also be volatile. Ellie has a love-hate relationship with John
Constantine (Reeves). PRINCIPAL ROLE

[Crazed Woman], any ethnicity, 20s - 30s. A neurotic, loud woman who
becomes the victim of supernatural forces. Constantine tries to help her.

[Attractive Girl], any ethnicity, 18 - 20s. Glamorous, flirtatious L.A.
model type.

[Church Attendant], any ethnicity, 30s - 40s. Old-fashioned, quiet,
level-headed, down-to-earth woman.

[Filipino Mother], Filipino, 30s - 40s.

Thanks to poster Adrian Brown at the insane rantings forum