Valve writer Erik Wolpaw recently explained why Chell, much like Gordon Freeman, is a silent protagonist.
Half-Life and Portal, two of Valve's most popular franchises, both star silent protagonists. According to Erik Wolpaw, in each game this is probably for a very different reason. He explains that in Portal, Chell is silent because it makes the game funnier.
At a recent NYU lecture, Wolpaw said: "There's this thing with comedy, there's sort of two different patterns. One is, you're the straight man in a world gone mad, and the other one is you're a crazy person in a sort of straight world. Portal is definitely the world gone mad straight man, and the straight man is you. And again, because you have to write in the margins in a game, time is at a premium, so the fact that there's already this established thing where you have a silent protagonist, that saves us a lot of time. You may want to know Chell's backstory, you may want to hear her say things, but I guarantee, if she had to say her straight man lines at the expense of half of the other dialogue, it would suck."
Wolpaw doesn't feel that people are actually invested in Chell as a character, and instead relate to the game world through her shell as themselves. Wolpaw admits that Chell can definitely talk, but added: "She just chooses not to, what with the robots all being dicks. Why give them the satisfaction?"
If you were hoping that more about Chell would be revealed in Portal 3, you might want to stop hoping. Wolpaw said that Chell's been through a lot after the events of Portal and Portal 2, and Valve just might "let her have her day" rather than bring her back again. At this point that's probably a good idea, because I'm sure there are hundreds of thousands of different ideas about what her personality is like by now. She could be a total ass, and that'd be a letdown.
The other most interesting tidbit that came out of the lecture was on a puzzle mechanic called "F-Stop," originally slated to replace portals. Valve worked on "F-Stop" for 6 months, but shelved it to bring portals back. Wolpaw didn't describe exactly what "F-Stop" was, though the term refers to a light regulating factor in photography also called the aperture setting (Aperture Science, get it?). He revealed that Valve may decided to use it in a future title.
So basically, if Valve is working on Portal 3, it could feature some "F-Stop" action along with a brand new protagonist. And he/she will probably be silent, because it makes for a better game where a sentient AI, or other disembodied voice, insults you.
Source: Giant Bomb