The core activity one needs to master to be a "good roleplayer" is making decisions and rendering the reasons for those decisions intelligible  to the other players.
If you have ever written dramatic works, you probably know that the progression through a character-driven arc is composed of a series of decisions which cannot be reversed or undone. The challenge of this sort of progression is to provide a sense of appropriateness and intelligibility to the protagonists and antagonists' decisions and their consequences. Successfully doing this constitutes "characterisation" properly understood.
The folk understanding of how decisions work is broadly that beliefs about the world combine with desires to motivate acts which are undertaken to bring about states of affairs that correspond to the satisfaction of those desires (this is called the "Humean" account in ethical literature). When we are trying to understand another person's actions (and do not consider their actions arbitrary), we try to reconstruct the above elements in a coherent way that is appropriately situated. We can do this retroactively or predictively.
When we are "roleplaying", we are conveying the necessary information to other players to engage in these kinds of reconstructions, and our success is determined by how well they are supplied with the information they need to do this.
We cannot ensure that they will reconstruct things identically to how we conceived the relation between the pieces of a decision, or even that they are particularly good at doing this, but this is an ordinary problem of intersubjective communication that we navigate all the time, and not peculiar to this situation.
The silly voices, tragic backstories, expository monologues, themes, tones, and the like are (often fairly crude) tools to convey information about the character's beliefs and motivations, and to aid others in predicting the kinds of decisions they are likely to make. We rely on tropes, stereotypes, conventions, history and other activations of the attribution bias to transform these trivial and incidental features into meaningful information.
But despite how obvious this sounds, I recurrently run into individuals who have all of the trappings of characterisation, but can't deploy them to render their decisions intelligible to others. For example, it's quite common to give characters a bunch of "quirks" that occur only sporadically when the player remembers them. They sit around, being invoked or mentioned but not motivating any behaviour or having much relation to the decisions the character makes. Then suddenly, the player becomes bored or thinks it would be funny, and the quirk is invoked in an inappropriate situation as a rationalising motivation for some bizarre action.
e.g. A character might have arachnophobia. Most of the time this quirk is irrelevant to what's going on and so is forgotten, but then one day, while the player is bored at the table, they recall that their character is afraid of spiders and decide to do something stupid like stab the innkeeper, justifying the action with "I saw a spider on his back" or some such.
The problem here is not having a fear of spiders per se, or even stabbing the innkeeper because of it, but that because the quirk normally motivates nothing, it has insufficient persuasive or explanatory force in reconstructing the decision for the other players when it finally does crop up. It just comes out of nowhere and is clearly a post hoc rationalisation, and not a very good one at that.
I believe having a large accumulation of trivial quirks like this is detrimental to roleplaying well, much like allowing a lens to become covered in spots and grime does not improve its clarity or focus. Either they provide too many possible rationalisations, diminishing the ability of other players to pick the right one to predict or reconstruct your motivation, or even if they can be successfully picked out, the actions they motivate are usually unreasonably intense (e.g. murdering someone) in comparison to the trivial and sporadic nature of the quirk.
The more quirks, the more facts about one's self that one can draw on, and the more random and incoherent they are, the more possible situations in which these quirks can be invoked (however intermittently) to provide a ready-made justification. But this renders the character unintelligible. Because the other people at the table have no idea which quirk will apply to which situation, and how tightly any of them will bear on a situation relative to the other quirks, the character seems inconsistent, unpredictable, and incomprehensible.
Most problems of this type rely on an over-estimation of the explanatory force of a character's facticity. That is, they expect a single fact, or very small set of them, about the character or the world to do the bulk of the work of justifying motivation and behaviour, often regardless of other relevant facts. "I am a chaotic evil orc assassin, so of course I stabbed the barkeep in the back". This sort of thing lacks a persuasive account of the desire at work and attempts to fill the gap (or cover over the actual desire) with some facts instead, which is unsatisfactory.
Against this, constancy, simplicity, and clarity of desires and sensibility and salience of beliefs are great aids in conveying a character whose decisions are easy to understand and predict. I think these values and the practices of roleplaying that display them are, if not underrated per se, at least badly taught in many instances where new people are being shown what roleplaying is.
One example of how they are badly taught is the value that is often placed on elaborate backstories for PCs. While by no means a universal value, I do think many people at one point or another come in contact with the idea that a good character has a lot of stuff happen before or outside of the game proper, which a good referee then activates in the game over the course of play. The backstory does the work, theoretically, of rendering the character's actions intelligible to others, by filling in various facts that situate their present actions as part of a larger causal chain. And, if kept simple enough, it can indeed help tremendously with this.
But the moment it requires more than a few sentences or a short paragraph to summarise, it's probably too convoluted and complicated to actually inform the other players' attempts to understand how it fits into the rationale for what you're doing. The more stereotypical and cliched, the longer it can be since we can rely on other resources (familiarity with genre and comparisons with other texts) to help understand it, but otherwise, I strongly suggest keeping it simple.
One perhaps inobvious paedogogical tactic that comes out of this understanding of roleplaying as intelligibility, is to teach people to clearly communicate why their character is doing something and only after they have mastered these kinds of rationalisations to move to conveying them through more coded means.
These can, initially, be relatively brief out-of-character statements, but you can actually get very good roleplaying out of people with even mediocre acting talents by having them provide these rationales in the first person, in-character, to the other players, especially if you have a couple of players doing this at once with one another. Instead of the cognitive load of a funny voice and a convoluted backstory, a player-character can devote their attention to persuasively arguing for their beliefs and desires.
As a supplement to this, you can bring things to a "dramatic crisis" as a referee by establishing the common and shared beliefs about the world that the debaters share near to the start of the exchange, thus removing the factual disagreements and forcing the PCs to talk explicitly about what they want with one another. If there's a trusted neutral party (a PC uninvolved in the debate) you can also have them do it, tho' I recommend also pushing this to occur in character and to be narrated in the first person. You can also do this with PC-to-NPC exchanges too, of course.
I've found that shifting over to this understanding of roleplaying from one that considers it to be more closely aligned with acting has been tremendously useful for teaching new players how to roleplay, especially since it does not simultaneously encourage them to think of campaigns as a type of prewritten story with a troupe of semi-competent improv actors assigned to figure it out. I encourage you to consider the above and determine its effectiveness for yourself.
"Intelligibility" as I'm using it here means the motivations and considerations involved can be successfully reconstructed by an external party in such a way that conserves their persuasive or explanatory force. It contrasts with two failure states that are distinct from merely bad decisions or decisions one disagrees with:
a) "Bizarre" or arbitrary decisions are ones that cannot be reconstructed, or that when reconstructed lack persuasive or explanatory force.
e.g. Stabbing an innkeeper because you're bored and want something to happen, or spontaneously deciding that you hate innkeepers and plan to kill them all, despite having interacted with many of them in the past in non-murderous ways
b) "Perverse" decisions where one makes the decision based on a different space of reasons than the one normally under consideration in play
e.g. Screwing over Jim's character because you're pretty sure he's sleeping with your partner IRL or, alternately, making a choice that boosts your character's mechanical power but involves introducing an explanation that can't be reconciled coherently with elements of the character you've previously established.
 There is a related problem in games where people scrounge around looking for the single fact that will trivialise or eradicate the need to make a decision for themselves even when no such fact exists or can be discovered by them prior to making the decisions. This involves the extension and repetition of a useful behaviour - identifying and eliminating from consideration undesirable or less-optimal alternatives using evidence - until it becomes detrimental, in a sort of speculative turn by which a thing turns into its opposite, The further the desire to eliminate all but some theoretical totally optimal and satisfactory option is extended past the point of possible knowledge, the more detrimental it becomes, until one can no longer understand the situation or progress through it to some other decision. This is an example of Hegel's famous "bad infinity".