Just watched the first movie again; I know that it departs on a few points from the book.
In Harry Potter’s first year at Hogwarts, Snape casts a counterspell to prevent Quirrell from causing Harry’s death in a fall from his broom during the Quidditch match, keeps Quirrell from stealing the Sorceror’s Stone after Quirrell lets a troll into Hogwarts, confronts Quirrell in the hallway one night and demands that he decide “where [his] loyalties lie,” and helps cast the various spells protecting the Stone against theft.
I wonder, though, why wouldn’t Voldemort (possessing Quirrell) reveal himself to Snape? Snape was a former Death Eater and was, as far as Voldemort knew, still secretly loyal to him. If he was in any doubt, Voldemort could have used Legilimency to probe Snape’s feelings towards the fallen Dark Lord. Snape would have been a useful ally at that point.
I thought that at the time, Voldemort thought Snape was a traitor. I can’t remember when he decides that Snape is on his side. Goblet of Fire maybe, at the end?
ETA: Snape was good enough to block Voldemort out of his mind, so presumably he always has some sort of defense up. It also seemed like Voldemort was only along for the ride; he couldn’t really do any magic himself.
[spoiler]This has always struck me as being one of the big, big plot holes. Legilemency isn’t the problem; Snape is known to be a master of Occulemency, and clearly can block Voldemort. The problem is that Voldemort seems physically aware of his surroundings there in the back of Quirrell’s head, at least at the end of Book 1, so he should know Snape is working against Quirrell, and yet it’s implied later in the series that Voldemort was NEVER aware of Snape double crossing him.
It’s possible Voldemort becomes more sentient (in the true sense of that word) later in the “Philosopher’s Stone,” after the confrontations with Snape and before his confrontation with Harry, but wouldn’t Quirrell mention to his master, “Hey, Snape doesn’t seem to be much of a help here.”?[/spoiler]
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Yes, Snape thought (and says as much) that he thought that Quirrel was acting to get the stone for his own use, not for Voldemort’s. He assumed V was dead or gone, like the other deatheaters. He(Snape) refers to this in book 4 or 5, IMS.
Book 6, actually, when Narcissa and Bellatrix come to pay Snape a visit and get him to do the unbreakable-vow thingie. Bellatrix is all suspicious of Snape and the whole Quirrell thing comes up; Snape explains he thought Quirrell wanted the bloody stone for his own purposes.
I think the most likely answer would be that Rowling didn’t have all the details of the entire plot planned out yet, so the beginning of the series may contain a number of plot holes in hindsight.
And also, by working against Quirrell, Snape was doing what Voldemort had asked him to do - keep Dumbledore unaware of his ‘true’ loyalties.
By doing the counterspell during the Quidditch match, Snape was keeping Dumbledore onside and also keeping Harry around for Voldie to dispose of, if he ever came back.
I’m sure she didn’t have all the side plots planned, but I think she did have the overall arc of the story planned all along. But, as pointed out by others, I don’t think this can be considered a plot hole, or even an unlikely stretch… Voldemort has no idea who he can really trust at this time, and unil he has power of his own it’s really in his best interest for no one to know his situation. Especially someone strong minded, like Snape, who would be more likely to kill him in his defenseless state than someone weak minded like Quirrel.
I seem to remember that in one of the books Voldemort actually mentions that he laments not trusting Snape enough to reveal himself to him. I can’t think of where in the story arc this might be, though.
Also the match wasn’t that far after Harry’s arriving at Hogwarts. Snape said he discovered Harry’s true nature “soon” after he arrived. He didn’t say how soon (probably deliberately).