Never say "think about what you want to say, before you say it." Never finish their words or sentences for them, unless they ask you to: and don't ask them if you need to, because they might feel pressured to say yes. Don't get TOO worried if they have times of quiet. Yes it could totally mean they're depressed, but it could also just mean they had a stuttery day even if you didn't know it and they just want to be quiet for awhile. Just let them know that you like them for who they are, and you accept them as they are. What they have to say is more important than how they say it, and that you can be as patient as needed.