First person always a player. As a player, conversation is always in dialog.
As the GM, first person always unless it is necessary to distinguish one NPC from another. Conversation is always in dialog.
It may risk my dice, but as a DM, I will insist that you engage other NPC with dialog. Summaries of what you intend to say to an NPC are not allowed unless it is repetition of something already said. I can't respond to a summary of intent, because it requires that the PC and the NPC have the same frame of reference, which they usually do not. A summary of intent in a conversation is as meaningless as any other summary of intent like, "I go to the end of the dungeon." or "I find the BBEG." or "I obtain a +5 Holy Avenger." The question of "How?" cannot simply be dismissed.
This technique was taught to me. I was mentored. When I first started gaming and DMing I didn't use this techinique with my middle school chums. Nothing was done in 1st person. When I later had the oppurtunity to play with older players, I was taught to use 1st person, and after initial confusion, stuttering and embarassment when doing so, soon realized how superior the results were in terms of emersion and enjoyment. In particular, I quickly realized how much more entertaining the DM was being by playing out the NPCs in first person than I was, and emulated this technique in my own game to much the same success. Thereafter, I have often noticed that groups which play in a first person emersive stance not only have more fun, but do a better job of imagining and interacting with the shared environment than groups that simply direct their characters like playing peices. That is to say, skillful power-gamers and role-players both do this, albiet for different reasons.
Lastly, as a kid who was borderline autistic through elementary school and often paralyzed by shyness, RPing in the first person was one of the most educational aspects of RPGs and in some senses a literal lifesaver. Nothing has been more important to my ability to obtain not only a normal social life, but even the ability to lead small groups than switching to a first person style of DMing. Ultimately, I took the skill of RPing out into the 'real world' and roleplayed being some one who could carry on normal social conversations. In this case, the appearance of being able to do something is much the same thing as actually being able to do it. And, as I've later taught many players to RP, I've spread the skill that was taught to me, and had the oppurtunity to watch shy, unconfident, stuttering players blossum in to full-fledged roleplayers. As a DM, that's awesome sauce, and one of the things that makes it all worthwhile.