In addition to being one of the five or six anime I will watch at any time for any stretch of time an infinite amount of times, and being just basically the shit in all ways, Gantz is a show that depicts people under pressure. Not the David Bowie kind either, serious life or death (after death) shitsnacks. Watch it, if you haven't. I won't give away too much of the plot, because I love it that much that I wouldn't want to fuck with your possible future viewings of the show.
Cast to... Sean Chiplock (Sonicmega)
"Oh hey, surprise surprise, sonicmega is in a dream-cast on the vaa derp derp." Yeah, I know. But I know Sean and have watched his talents develop and if there's something he has a mighty gift for, it's angst, and Kei is possibly the most angsty character since Shinji Ikari. Before the events of the show unfold, he's an angst ridden student who wants to get laid and is a slave to hormones. Once the events begin to unfold, his angst basically explodes out of control and he starts to get a sociopathic streak that he finds frightening. Because we live in Kei's head for a plurality of the show, we hear exactly how much he yells at himself and all the pent up rage he holds inside. Sonicmega was born to depict this guy.
Cast to... Adam Tilford (Adam)
Kato thinks of himself as an older brother to everyone (except Kei) and tries to be as protective as he can be. While good, through and through, no good deed goes unpunished and Kato finds himself pulling the short straw in life far too often. He bottles up his feelings of rage, and normally has a complete hold on them, under the surface, but when pushed, he can fly into fits of rage that are nearly murderous. Adam does an excellent job at making his character someone you'd root for, whose team you'd want to be on. While Kato can be a little too naive to be likeable at times, I can see Adam finding the right moments in which to make his stoic front ebb away, and to show exactly how much this guy is hurting.
Cast to... Sheepy
Kishimoto is, at her core, impotent. Unable to completely get past the SPOILERSPOILERSUICIDESPOILER that led her to Gantz, she finds it almost impossible to take action. She has a very bleary view of the right thing to do, but certainly knows what's wrong when she sees it. While a mostly intrinsically moral character, she doesn't do a whole lot but sit back and watch as those morals are either substantiated or trampled on. She is moved by emotions and not reason. Sheepy, whose work I first heard when I first showed up on the VAA long(ish) ago, has a background in live theater and an excellent touch with this sort of character, a girl who is lost and vulnerable, in the wrong place at the wrong time. She'd do a fantastic job making each cringe and tear be felt by the audience.
Cast to... Y. Chang
By far the most fun of the characters, Nishi is a young sociopath who takes extreme pleasure in the activities Gantz forces upon its participants. More than that, he is a solipsist, whose flawed concept of the world is actually proved *correct* by his relocation to Gantz. Because life is a game to him, he has fun when he's winning and gets angry when he's losing. Deep deep inside, there's a very sad child who just wants affection. But that's under a mile-thick blast-door of pure evil. I picked Y. Chang because I've never heard him in a role as crazy dark as this and think he'd do a badass job. Nishi has several points where his too-cool-for-school demeanor turns into something monstrous as well as one where you get to see his hidden vulnerability - Y. Chang is a good actor and a clever guy, I can see him voicing this very creatively.
Tetsuo the Biker
Cast to... D-Mac
A biker named Tetsuo in an anime? Whoa! Anyway, Tetsuo is in his 30s, and is the alpha dog of a biker gang. That being said, he's probably the only one among his crew with a heart, and that heart is in the form of his wife and kid. Though he has a tough exterior, at the end of the day, he's kinda a softie. He's deep in debt and is always going to the store to buy milk for his infant son, because his wife can't produce any. D-Mac can be both tough and cool and caring and fatherly, and is a pretty solid choice for that reason. Plus, he's an experienced guy, and Tetsuo should definitely have a very strong air of experience, despite being shoved into a new and crazy situation.
Cast to... Sapphire
She's the first example in Gantz of a female who isn't a bitch or a victim. She's got a tender side, but tends to be more strong and silent. She's an optimist that acts jaded and cynical in social situations, but more than anything she's determined. Cast to Sapphire because guuurrrrrl. Oh right, a real explanation. Well, Sapphire has attitude (hell, she cast herself as Kim Pine in this thread), and that'd definitely come through as Sei. In addition, it'd give me a chance to see how she does with the occasional sappy inner-monologue that Sei has, and I think Sapphire would keep a good balance between the attitude and compassion, and not go too far with either quality.
Hojo the Flam- er, FAMOUS person
Cast to... Tomamoto
Hojo is vain vain vain. He's so into himself that most characters assume he likes all men. All he cares about is his image, but once in Gantz, he learns to adapt and follow orders faster than most, and is the first person of his group to SPOILERSPOILERSPOILER. Survival instinct he's got, even if it comes second to his ego. He's also kinda an airhead. Well, mostly an airhead- not in a funny way, just in a very shallow empty way. Tomamoto is the perfect choice for him, because not only can he so amusingly get up in arms and defensive, but he's also really hysterical at depicting people that aren't too clever. Though there isn't too much *acting* involved, Hojo does eventually show a more serious and lovable side that Tomamoto would be very adept to switching to.
That's really all I can think of for now. I'm not gonna cast anyone in the anime finale-arc, though that would be tempting.
But above all, watch the damn show, it's great.