#especially if someone else doesn't know marty - it's just one of those amusing things

[doctorbrown]

I think any time anybody calls Doc Emmett—you know, his name—over the formal Doctor or Doctor Brown, it throws him for a loop.

And not always in a bad way; it's just surprising. Like oh, people remember I have a name and they use it. Wow. Doc's not so deeply attached to his title where he'll get fussy if someone calls him Mister Brown, for example, but save for Clara in canon and maybe Lorraine and George in the LP timeline later on, given that I personally think they have a closer relationship with Doc compared to the one they (didn't) have in TP, nobody particularly calls him by his name, Emmett.

If Doc makes the effort to correct one of his peers in a way that doesn't have an underlying reason behind it, emphasising that it's Doctor Brown, then that's absolutely a sign that his opinion of you is startlingly low and he won't tolerate kindly being called anything else. It's a distant, strictly formal relationship.

He does still appreciate the nickname Doc, though, and always will. And it's always amusing if/when someone else chooses to call him Doc (he allows this if he likes the person, why not?) because he'll always be reminded of Marty suddenly calling him that for the first time.