BioWare's Daniel Erickson has said that during the single-player or solo portions of Star Wars: The Old Republic, a friend can join you on the quest, but they will not be able to mess with your story arc.
Speaking in an interview with VG247 at E3, Erickson said that the multiplayer part has not been seen by many people yet, but "world arcs are huge" and there are "double-digit hours" worth of quests in multiplayer.
"That’s the part that people really haven’t seen much of yet," he said. "They’ve only gotten to see the origin worlds. The origin worlds are very story-focused, so a lot of the content – although your friends can come with you and play with you – is still really like 'You’re Luke and you’re dealing with Yoda.' And yeah, you guys can team up together and you can go and help him out, but about half the content there is solidly single-player.
"That proportion goes way down as the game goes forward. Again, it’s not single-player, per se. Your friend can go with you, but he can’t mess with [the story].
"Later we get into the multiplayer stuff, and that’s what people haven’t gotten to see. The world arcs are huge. They are double-digit hours of quests – giant chains, all built for multiplayer, all built for your whole group to do this great story together. And that’s where you get the actual story of the war. That’s where you get the conflict between the Republic and the Empire. That’s where you’re trying to take over worlds.
"That’s where you’re changing history".