I have not watched CR; fair warning, full disclosure, and all that.
IME most people can come up with a good one-liner or two during a session, but expecting them to act all the way through is more than they can achieve. If somebody does 'get in character' for a while, it is memorable.
When I DM'ed Tiamat, I gave Arauthator (ice dragon) his own voice, he knew secrets about the characters, he was a tactical master. I had a month beforehand to think about it, as the PCs worked their way through his lair over several sessions. Fighting (and defeating) him was a high point in the campaign.
For one-and-done cannonfodder foes, no such work went into them. They had a bit part and left the stage, not really noticed. They were part of 'the world is going crazy and some hero needs help to get a grip on things again'.