It really annoys me when people act like Alistair and Cullen are the same character, when they are very different.
Alistair grew up with child neglect. When visiting Denerim, Eamon kept him in the kennels. At Redcliffe, he slept in the stables on a pile of hay. Alistair also recounts a time when he was locked in the dungeons for a day before someone came to get him out. And of course he also talks about how Isolde despised him, and “made sure the castle wasn't a home.” But is still convinced that Eamon is a good person and he deserved all that. Cullen had a very fortunate upbringing with a loving family who supported him and what he wanted in life.
Alistair never wanted to be a Templar; he was forced into joining the Order by Eamon. He is vocal about how much he despised this, and considers Duncan recruiting him for the Wardens as “saving” him from them. The only thing he says he enjoyed about Templar training was the educational component, which he did not receive previously. Alistair was a poor recruit because he frankly did not want to be there, and therefore did not take it very seriously. He saw practices like the Harrowing as horrifying, and deepened his dislike of being a Templar further. And as time goes on, he becomes even less of a supporter of the Order; he outright says Meredith is the biggest threat to Kirkwall in Dragon Age II, if made king of Ferelden. It was always Cullen’s dream to be a Templar, and would even force his younger sister to “play the apostate” for his “training” before being recruited. Cullen was an enthusiastic recruit who considered Templar training “all that he had imagined”, and “did not hesitate” in taking his vows. Even the Harrowing did not waver his devotion to the Order, which by Dragon Age II becomes downright fanatical and tyrannical, practically worshipping Meredith. (Though this was later attempted to be retconned in Dragon Age: Inquisition… just as poorly as all the other retcons in that game, taking the path of “just pretend he never said and did all those things!”)
There is a lot of dialogue from Alistair about how much he dislikes the Chantry. Cullen, on the other hand, is extremely faithful and the only criticism he ever has about the Chantry is that they don’t treat the Templars well enough.
Alistair has a good sense of humour—in fact, it’s one of his biggest coping mechanisms. Cullen wouldn’t know a joke if it hit him in the face.
The player can disagree with Alistair on every turn. He is presented as sometimes being right, and sometimes being wrong, like most people. (Side note: more than that, you can be downright verbally, emotionally, and physically abusive to Alistair. Holy shit, I didn’t even realize how bad it can get until reading through the dialogue in the toolset, because I’ve never picked those options in game. I was honest to god flabbergasted and very uncomfortable through much of it.) The player rarely has the chance to even mildly disagree with Cullen. On the rare occasion you do, the dialogue is painted as if the player is being an unreasonable asshole, and he never even addresses what they say. (Example.)
The only reason I think people are capable of mistaking them for another is because fandom likes to donate Alistair’s personality onto Cullen. That and the the ever-frequent whitewashing of Alistair doesn’t help matters. But I’m not even a Cullen fan and I think it’s a disservice to both of them to act like they’re just Alistair and Alistair 2.0, honestly.