In an interview with Destructoid, Producer of Mass Effect 3, Casey Hudson explains how decision making effects the series –

“With the first one, we’re kind of launching everything, and we’re kind of throwing up these huge threads to the later stories,”

“And then with the second one, we’re kind of in between. It’s actually the toughest one to make, because it has to work with previous stuff and still tie in with other stuff a little bit later."

“But with Mass Effect 3, this is really the opportunity to do whatever we want with the reminder of the story – which means players can do whatever they want with the remainder of the story. Your decision can have much bigger consequences, things that you’ve done earlier can have much bigger consequences.

“So we can really throw around a lot of big impacts from your decisions, from prior games and from decision you’ll make in Mass Effect 3.

“It’s really astronomical. The number of different ways that you can play it, the combinations of different decisions that you can make, and then all the different ways that the story can end up depending on what you’re doing.”

-Christian "G+" Spring

IGN

Eurogamer

Destructoid