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.