One of the characters in F & F (Flame and Filch) is very old, very bright, very male, and can't speak a word (ladies and gay gentlemen, feel free to make comments about him being the ideal man). I've had a lot of fun coming up with dialogue for him. No, really. I have. Just because he can't speak doesn't mean that he can't communicate. In fact, he has a lot to say.
So, how did I solve this silent problem? Well it was as hard and easier than you think.
The first possibility that came to mind was to use ASL or a form of it. The problem with that though was how the heck to describe the hand movements. Sure I could have described every finger wiggle and accompanying expression, but a single line of dialogue described in that manner would take an entire paragraph to complete. Plus it would be incredibly confusing if the reader didn't know ASL. I'd hate to force them to have a picture dictionary nearby just to read a conversation, so that possibility got nixed.
The next possibility I thought of was to use telepathy. It's an easy and well established method in fantasy novels so I certainly wouldn't be confusing any readers. However, I also wouldn't be doing anything original. I didn't like this possibility because I felt that this was one of those situations where taking the super easy route would hurt the story. All those italics would make it very obvious that there was dialogue on the page, which may cause the reader to skim, and that would be bad. That doesn't make this option weak -- in fact I've read books where it was used really well -- it's just not a device that I can use heavily.
What I finally decided to use was body language and to treat that the same way I would dialogue. Everybody uses non-verbal cues to communicate -- a nod, a wink, a shrug of the shoulders -- and it's something that's already incorporated into my stories. Granted, I'm more accustomed to accentuating dialogue with it, not replacing it. Non-verbal cues also have a similar problem to ASL in that I have to quickly and clearly describe the gesture or expression, while making it unmistakable what he said.
Describing the non-verbal cues succinctly was easy enough since it's typically one or two motions. As for the rest, I discovered that the dialogue was easy enough to understand because we already know what certain cues mean. A wink = I'm being playful. Slumped shoulders = disappointment. A nod = I agree. I didn't need to add anything to it. It's clear enough on it's own. The one thing I have to be careful of is to not sacrifice his strength for a conversation. Every non-verbal cue that I can think of is used in reaction to something, whether it be something someone said, or a situation, it's still a reaction. I have to walk the fine line of having a reactionary character without making him weak. I may have to incorporate some selective, magical telepathy in order to give him an opportunity to be an instigator. We'll see. For now this method works well enough.