Non-player characters are one of the trickiest parts of gamemastering. GMs who are strategic and tactical geniuses, who can handle a combat scenario on three fronts and two different timelines, will stutter and blanch and croak out some garbled phrases in a painful Cockney accent whenever their players meet a new person with something to say.
Short of taking acting classes, there are a couple of ways to make these encounters less stressful for the busy GM, while giving the players the chance to interact with the NPCs that have the info they need.
First, if you can't do accents, don't. What the NPC has to say will, in virtually all cases, be more important than how they say it. Bad accents do make everybody laugh, so there's a place for them, but if the in-game exchange is important just use your own voice with appropriate emphasis and emotion. You can do a lot to distinguish NPCs just with wording and volume, and maybe a facial twitch or figure of speech. An innkeeper who says "Bokram's beard!" every time the players are in his bar is memorable, and when they need information in the future they'll remember him. Especially if they hear about a cult of Bokram operating in the area....
Second, don't try to improvise vital exchanges with NPCs. Write down what needs to be conveyed to the players and keep it front of you, either verbatim or in outline form, and refer to it while the conversation is taking place. Remembering 2 hours later that you forgot to mention that the Guardian can only be harmed by the Vrakas Dagger after it has been immersed in the blood of a basilisk....well, it's embarrassing at best and logistically nightmarish at worst. Save yourself the last-minute scrambling and write it down.
Finally, if you and/or your players are not comfortable with speaking in character, just don't do it. The second-person perspective is a time-honored and completely valid way of role-playing your NPCs and characters. It removes the self-concious fumbling after words that dilutes the interaction and slows down the action. RPGs are games, they're supposed to be fun. Forcing players to do something they're not comfortable doing can ruin their gaming experience, and it's just not worth it.
That being said, sitting back and watching the PCs interact with each other in character can be more fun than a cat with tape on its paw.
Speaking of NPCs, there's a Notable NPC generator in the Dungeon Crawler's tavern maker that churns out descriptions of interesting folks for players to encounter, including names and personally identifying quirks. Just saying.
PS Please, oh please, if your NPC or PC has a speech issue of some sort, especially a stutter (and you don't), for the love of Bokram, don't roleplay it. Your friends will thank you.