You could also add some accents to that, so if you think of signed as the equivalent of said, you might talk about tone of voice in how the fingers are moving - spikily, fluttering, quickly, slowly, etc. I'm not being super specific here and obviously you wouldn't want to do it all the time as it would be too much, but instead of the character 'shouting' at someone, they might use staccato hand movements to indicate their displeasure.