Over time, you could begin to frustrate your friend with your frequent criticism of the ways in which they think and express their ideas. In conversation, you'll pick at their logic and expose the flaws in their thinking. In your mind, you'll be 'helping' -- pushing them to think more deeply and carefully rather than skimming the surface of an idea.
Similarly, when they seem indecisive, you'll push them to choose a practical course of action based on the alternatives available. But without realizing it, you'll be reacting not to what they've truly expressed, but to what you think they said. They'll feel as though you haven't even heard them, and they'll be right. Instead of perpetuating this situation, let them think and talk in the ways that are natural to them. If you can do that, then this dynamic between you will have a positive outcome: that of teaching your friend to think more carefully and thoroughly, and teaching you to be less critical of others' ideas and self-expression.