Sorry, but I love DB's acting and his reactions. He's actually very expressive right from the start.
That's why I'm constantly infuriated by people saying he doesn't smile or cry, because I can give you photo after photo proving it wrong (including with Buffy on BtVS). Angel also always had a self-deprecating or black dry humor (DB excels at dry humor and that has always been his style). It's false that he was just a boring brooder in BtVS (such garbage) that didn't emote. While he doesn't hyperventilate (like Alyson) and he's more stoic with quiet suffering, he still does emote a lot.
He was instantly one of the most interesting parts of BtVS right from his first scene and it's because of DB's charm and charisma that was there right then. You knew immediately that the other guys so far introduced didn't stand a frickin' chance. It was more than just his looks. He also was the mystery to be solved. DB very much played that up by teasing Buffy and the audience with his expressions and sly humor, but also previewed that there was more going on with him on a more serious level very early on (The Harvest).
I HATE reading about SMG carrying scenes, because it's NOT TRUE. This just has been repeated so often by Spuffy 'shippers who hate DB and Angel viciously that even Bangels feel they have to be apologetic about it and start saying the same things. DB never gave a horrible performance (I can think of other actors who did--CC in Tomorrow is a big one).
I actually find myself sypathizing a ton with Angel because of how he's performed. There are lots of subtle things in DB's performance that are just beautiful and very emotive. He shows amazing vulnerability and tenderness for such a big Alpha jock of a man with a sweet, soft voice, while also being able to turn on a dime into incredibly intimidating, powerful and dark. And no, I don't just mean just his Angelus performance; this was already present as of 1x07 when he monologues and came out several times even before Innocence (What's My Line? comes to mind). Angel was always presented as being a bit more than he might have initially seemed. Even his more confident first appearance turned out to be a façade for a deeply broken individual by the second episode.
He gave many great performances right from the start. 1x07 is the episode that pretty much figured out what the show was doing and elevated it beyond Goosebumps. DB deserves a lot of the credit for being a big part of that. The WB was thrilled by him and Bangel. This is all part of the post-series rewrite to minimize the fact that he was a massive part of the show's initial success and it wasn't actually so shocking to everyone that he was given a spinoff (contrary to this current received wisdom that nobody liked the character and everybody thought he was a horrible actor), given he was the most obvious character for it with an actor who so quickly proved he could lead many shows.
Some people seem to equate being the equivalent of the Fonz with charisma. This idea that only big, flamboyant, showy characters are the only ones with charisma is quite wrong. Those characters are mostly sidekicks, not leads. Both BtVS and AtS suffers from a fanbase that can't figure out who the dramatic leads are and why the shows have to be about them instead of the funny sidekicks.
David Boreanaz was called on to give serious performances with a ton of pathos, not just big audience-pleasing comedy sidekick performances. And he had to actually act for the part of Angel, since he was not just playing himself as so many do.