"Not to dismiss Mr. Furlong’s performance in T2 but the character of John Connor was always abstract to me until I watched T:SCC. Mr. Dekker brought the character alive, all abstractions blown aside. (That kid will win an Oscar someday if he continues to act.)

In S1, he’s a scared kid who is “not a messiah” and begs his mom to stop it. (I’d imagine J-day is inevitable based on S2’s finale) His mom agrees and off they go; along the way we see the growth of JC through to S1’s finale: his compassion for the suicidal girl; his abiding love for his mother even when he finds out she was ready to give him up; his clarity regarding Cameron “the only way I’m reassured by that is that in the core of your chip you’re just like him”; his love and trust for Charlie; and his newfound relationship with Derek.

In S2, there is a distinct change in his personality. I’d imagine killing someone would do that. JC starts making real decisions that affect everyone around him. He also realizes that his mom can’t protect him either. He killed Sarkissian, not Sarah. He tells her, “we’re not murders; you’re not.” He evades Cromartie without anyone’s help at the pier. While all of them including Cameron go off the rails, he continues to see clearly. He knows who Riley is. He figures out who Jesse is and that lovely double entendre at the beginning of “Today is the Day, Pt2” when he tells Derek he wants to talk about Derek’s future; oh so subtle.

I don’t see him crying over Derek’s death because future John would “beat his a**” for that. I think he realizes more than the others that J-Day is inevitable. I also think that had there been a Season 3 we might have seen just why it is he who is the leader of the resistance. I think his ability with computers is an unknown quantity.
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