My memory on the TV show is rather fuzzy, but I remember liking the way Cassie acted just fine. My mental image of her was pretty much exactly how the model looked on the book covers, though, so since the actor playing her didn't resemble the model exactly I couldn't call her the "perfect" Cassie... but her acting was well done. I remember NEVER liking how Marco was done, though. He looked wrong and never acted like the easy-going Marco in the books. (I know actors don't always match the appearance in the original works, so perhaps saying he looked wrong is a silly complaint...but aaaah I never liked how he looked at all :|)
As for Rachel's actor, I do agree that she never really seemed tough enough. Maybe if the show could have gone on longer she would have improved, but I'm not sure. It wasn't the acting that was off so much as how she presented herself, if that makes any sense? Like, body language and all that stuff. I'm sure the actor tried her best and she's probably a fine actor--uuuh I haven't seen her in anything besides Animorphs so I can't properly judge ha--but I doubt she could ever have that tough, confident air that Rachel always had. Rachel was a tough, bold person from day one before the group even became Animorphs, and even if the actor had the right "looks" to play Rachel, she just never presented herself as someone bold like Rachel.
(I think I actually vaguely remember the Jake/Rachel stuff on the show. Pfffffffft I completely forgot about it that's kind of hilarious.)
