He has an interesting voice.
I've only seen two movies with him, Harry Potter 4 (in cinemas), and Troy (a couple months ago).
When I saw Troy I wondered: "Where did I hear that voice before?"...and I actually had to look it up here on IMDb in order to find out. That's when I realized, "Oh yeah! I remember Moody"
I guess it was hard to recognize his voice in Troy because his first scene is so much more calm/peaceful, whereas in Harry Potter 4 he's all "grouchy"...Either way, it shows how much range Gleeson has.