If I had to guess I would presume it's because when the scene starts, the character hasn't been speaking, so the speaking_char is probably either None or someone else. Then when the line starts, the character is then set as the speaking_char, so he lights up to normal. If you want them to start off already focused, you can manually set the speaking char by doing:
$ speaking_char = "guard1"
either before the transition or sometime during it. Which would then have the sprite start off in the talking state.
Hopefully that helps. But if it's some other issue, let me know and I'd be happy to help correct it.