I got the idea from that scene where he's sitting on Charlie's bed. There's a second that flashes by where he isn't smiling at all. When he said "... dooming everyone you love ..." He frowns and the lines under his eyes appear even though the frames before don’t have them.

Since noticing this I started watching all his little expressions and this is what I found.

Eyes

- The line under Alastor’s eyes will deepen or go away completely depending on how strained he is.

- His eyes will narrow like a snake when he's scheming. It can look like anger but it's not. When he's being kept by Vox his eyes are most often scheming.

- Anger looks more flat and irritated - which happens more often for him than all out rage.

- Rage only happens a few times and looks spiky. His pupils get very small and he can go cross eyed.

- Eyes go very wide and can point in different directions when he's slipping into madness, which is closely tied to his ambition and vision for the future or his enjoyment of violence. It happens when he's fighting Sir Pentious and when he's singing the neon part in Hell's Greatest Dad.

Mouth

- Smooth V shape when relaxed or happy

- Snarls when angry, unhappy, or disgusted

- This can look like the snarl, but the curve is at the front of his mouth rather than the side and it happens when he's mocking someone or being devious.

- He can be sad or worried on rare occasions and his eyes will go slack and droopy. You can see a lot of eyelid. This is pretty similar to ...

- Affection! His eyes stay round but they close a bit and you can see a lot of eyelid and rounded brows. This usually happens when he's looking at Charlie (same for sadness) or Mimzy.

Alastor slides through a lot of big emotions whenever he's on screen. He's most often scheming and devious but it can be combined with a ton of other things. He has a sort of openness and reactivity that makes him seem boyish. He's quite sensitive and anxious in temperament which he works hard to hide. But he also can't resist trouble and is almost always being cheeky and unserious. I think he has moments where the madness actually takes over and he doesn't always have control over the extent of his actions. It's odd because he can be the smartest character who's holding it all together and guiding Charlie but he's also incredibly immature as well ... I guess he did die young ...

It's all stuck in my brain. I've had this gut feeling that Alastor is a secret good guy.