Recently, actress Jennifer Hale, who voiced the female version of the protagonist in Mass Effect, shared her memories of working on the game.
Most often, the actors read from a script - they learned their lines during the recording:
85% of the time [voice actors] work cold reading [without prior knowledge of the script]. We never see our lines until we go to record; we do a couple of sets of three takes, a few tweaks, and then it goes to market. So when I played Mass Effect, I had a lot more context available to me. I was shocked and thought, "Oh my god, if I had known that, I would have added nuances and that kind of emotion."
Nevertheless, the creators of Mass Effect still provided the actors with certain information - a brief overview of the plot, the world, the development of the characters, an explanation of the branching storylines and dialogue options.
First, Hale recorded a neutral version of the line, and then added nuances for the Renegade and Paragon options:
It was juggling a bunch of details in the moment, but that's what recording for games is like. [Because of this "juggling"] emotional ups and downs are your normal workday. I always joke: Paragon is who I want to be, and Renegade is what I want to say.