random thoughts:
Diego Luna acting as Cassian Andor doesn't dive headlong into his lines. To me, he can convey a deeper inner monologue. I can't fault anyone for whom the same dissociative behavior simply comes across as vacuous instead, but all I can say is that it works for me.
As an interesting side note, some actors (even mediocre ones) have an innate quality that can project layers of introspection, whether intentional or not. Marlon Brando was a legendary method actor who had interesting cadences and breaks in his dialogue seemingly driven by an elaborate emotional narrative. He would later confess that a lot of the time he was pausing because he was trying to remember his line. I don't know if he was just pulling our leg because he just didn't want to talk about his process (which wouldn't be unlike him), but I think it's just as plausible that it is in his bag of tricks to manufacture the effect.
In Rogue One I never thought Diego Luna was "boring." e.g. I really liked his delivery of, "I've been in this fight since I was six years old!" When he was confronted by Jyn about his mission to assassinate her father, Cassian was uncomfortable and trying to avoid confrontation by being dismissive. At the same time Luna's acting conveyed inner conflict that he still wasn't sure if he made the right choice by not shooting Galen Erso. Now that I think about it, you can say that up to the very moment where he decided not to pull the trigger on Galen Erso, Cassian had been operating with the moral expedience of his mentor, Luthen. This moment in Rogue One would have been a turning point where he is inspired to become more than Luthen and start to have faith in people and follow his humanity again.
Another side note. Andor indirectly addresses the apparent discrepancy with that line in Rogue One about him being in the fight since he was six. Clearly, the truth is that he's actually been avoiding "the fight" for most of his life. And the empire wasn't even around when he was six. Andor addresses this by showing how, under stress, Cassian will stretch the truth to cover up weaknesses in his character. In episode 4 he tells Luthen he fought in Mimban for two years when he was 16, but Luthen reveals he was actually a cook for 6 months and didn't fight but ran.
So in Rogue One he was simply exaggerating to Jyn in a verbal retaliation to being cornered by her: "Suddenly the Rebellion is real for you. Some of us live it. I've been in this fight since I was six years old. You're not the only one who lost everything. Some of us just decided to do something about it."
