This episode is a showcase for some great acting by Amanda Tapping. There are a few 'phases' of her character and she plays each one differently. The first 'phase' is after the teaser, when Carter has been infected by a goa'uld but before the characters know about it. She remains aloof and slightly out-of-character, exactly as if she were being played by another actress who didn't fully understand the Carter character. Even in the background of scenes where she doesn't talk, Sam is seen staring into space, not giving her full attention to what's happening.
The second stage is once the other characters have found out about her infestation by Jolinar of Malkshur. At this point the audience and characters know as much as each other. Jolinar is portrayed as a stereotypical goa'uld - arrogant, mean, and tricksy. Thus, when he claims to be a member of the Tok'ra (anti-System Lord resistance group) Jack and co. are loth to believe him. It's only after the ashrak tries to kill him (and Carter in the process), and after Jolinar dies saving Sam's life, that the characters realise they could have trusted him, after all.
It's after this that we see the third phase of Carter - emotionally unstable. Nobody else can begin to imagine what she's gone through. Having another personality in your head, mixing with your own, only to have it taken away from you through torture, it must be distressing to say the least. The start of the healing process is shown in the final scene, when Cassandra (from season 1's Singularity) comes to visit Sam in the infirmary and comfort her.
One thing I don't think is quite right with the episode is that Jack doesn't seem concerend enough when he learns there's a Goa'uld in Sam. The same thing happened to his friend, major Kawalsky, who died as a result of them trying to take it out (The Enemy Within); for all he knows, the same could soon happen to Carter, to whom he's as close as, if not closer than, Kawalsky. Sure, he's seen Kendra (Thor's Hammer) and learned that safe extraction is possible, and he's keeping that hope alive for Skaara and Sha're too, but the Kawalsky situation must have been going through his head at the time. He does show more of that soap-opera-level emotion later when Carter's on the operating table with Janet trying to save her life, though.
Speaking of Sha're, we're both reminded of her (which is really the driving force behind SG-1's exploration of the galaxy) and given a tidbit about story development, as Jolinar tells Daniel he knows where Sha're is. Of course, it's entirely possible he was lying, but seeing as he's dead by the end of the episode it seems we'll never know. Something tells me it won't be long before she shows up again...