Hey so this is a question about the usage of minor illusion to replicate sombodies speach. So my plan was to use disguise self to optically look like an NPC that Ive observed and use minor illusion ( since its confirmed it may replicate speech) to sound like him and thus impersonate him. However the GM was unsure of this combo and said, since minor illusion is Somatic, i need to make gestures that would be suspicious before every sentence i cast. For mages the constant casting would be obvious anyways but for non mages i thought my character could use his deception to disguise those as natural. Anyways we thought it woupd be best to ask people who have nore experience on this. 

Any help is appreciated <3

Some DMs are for this because they see it as encouraging clever roleplay.

Some DMs are against it because there are actual mechanics for removing spell components like the Subtle Spell metamagic, and giving those features to anyone with a good Deception check or whatever undermines the value of those mechanics (as well as existing mechanics for imitating speech, for that matter).

I do think that using MI to simulate your half of a whole conversation without being obvious would be stretching the limits of what the spell should do, and it wouldn't really cover synching your face movements to the words which people are generally pretty attuned to.

Thanks! Definitely understandable and logical! 

You wouldn't need to do it before every sentence. The spell lasts 1 minute, so you'd need to do it every 60 seconds. Is that better? I mean, I guess...

You can definitely sound like him when nobody's looking at you, and you can definitely look like him when nobody's listening to you. That might be enough, and whether it is or not, I imagine it'll be a lot more amusing for everyone.

Edit: Any changes your character might make to the somatic gestures, in order to pass them off as just a strange quirk or something, would render them ineffective. You either do them or you don't -- there's no halfsies, in my reading. There's also no detailed description of what they are, so one could argue the gestures could be quite subtle, but the fact that they exist at all, to me, suggests that they're not meant to be ignored by NPCs. If I wanted players to have an illusion spell they could cast in full view of NPCs and nobody would notice, I just wouldn't give it a somatic component. If it was really so important that they just HAVE a free hand, even though they're not USING it for anything, then I'd include text to that effect: "You must have a free hand to cast this spell."

In general the rules indicate that Verbal and Somatic components are distinctive enough to be noticeable to an observer, although for something like Charm Person I'd apply the Jedi Mind Trick principle if the target fails the save they don't realize what's happening. Ultimately, it's a DM call, but I agree that given that the metamagic option to remove the components is called "Subtle Spell", you can't really just twiddle your fingers, whisper under your breath, or otherwise hide the components from an aware observer. If you want sounding like someone else to be something your character can consistently do, there's the Actor feat. Really, one good rule of thumb for "can I do this?" questions is to look and see if there's a spell, feature, or feat that covers that effect. If so, you're kinda pushing the envelope to try and get something close to that effect purely as a request.

+1 on the Jedi mind trick. If the spell succeeds in charming someone, I think they do whatever mental gymnastics they need to do in order to justify it to themselves. Maybe "I was being aggressive, they clearly just charmed me to calm me down," or "it's not really bad to charm someone as long as you're not doing anything evil," or y'know, whatever. Some creatures and people wouldn't really know what a charm is or what it looks like when someone casts one.

But anyway this is about illusions! Minor Illusion isn't a save-or-be-fooled spell, it's a be-fooled-or-save* spell! (*Actually an ability check, not a save.) Even if they saw you casting and they knew which spell you were casting, they'd still perceive the voice as real, and as coming from you. (Provided that you move your mouth properly, and idk what a DM would decide to have you roll for that, so let's just say you can.) They'd just have their suspicions, and could try to study your lip sync or whatever, as outlined in the spell description.