They used Paul Walker's brothers, as explained in [this article](http://www.buzzfeed.com/adambvary/the-daunting-task-of-making-furious-7-without-paul-walker#.onXGaKY4L):

> To double for Walker, the production turned to his younger brothers Caleb and Cody, as well as actor John Brotherton (who also has a separate role in the film). Visual effects artists then replaced their faces with Paul Walker’s face — but occasionally, Brian O’Connor’s lines would be a hybrid of multiple voices. “Sometimes, we would use the brothers to say certain lines, because we wouldn’t necessarily have ‘Archive Paul’ saying that,” said Wan. “So we may have the brothers saying one or two lines here, and Paul would finish off the sentence. It was this incredible jigsaw puzzle that we had to put together. I mean, Paul had a very distinct way of speaking. [Cody and Caleb] have the mannerisms, but they don’t sound quite like Paul. So there’s a lot of tweakage that we had to do. It really is a combination of so many different techniques and different methods to make it work.”

Yes, I know the link is from buzzfeed, but it is actually a good article.  Unfortunately, it does not reveal *which* scenes, which [I am trying to find out myself](http://movies.stackexchange.com/q/39162/49)!