Role-playing

It's more than doing "the voice".
The game Dave Arneson and Gary Gygax published as Dungeons & Dragons in 1974 had grown out of their experience with wargames. At first it was man-to-man conflict with a referee or judge to apply the rules and keep the game moving along. Figures received names, and therefore the beginning of personality. Mr. Arneson came up with a mechanism (or three) for advancing the power level of figures who survived battles. Experience with the game Diplomacy had given many of the players practice in "speaking in character", negotiating alliances, making deals and threats, all as part of a game. The self contained dungeon exploration game idea was also circulating at the time, as evidenced by it being talked about in some of the fanzines. Gary Gygax and friends brought these ideas together in the original little brown books and sold them as the world's first published role-playing game. The game quickly gained acceptance and a new hobby began.
Learning to play the game in the mid 1970's is commonly held to be nearly impossible without someone who is experienced with the system acting as teacher for the uninitiated. The rules presented in the original three little brown books are somewhat cryptic and incomplete, requiring additions in terms of both mechanics and insight. Rather than being a completely negative feature, the freedom offered by their being suggestions and guidelines and somewhat incomplete ones at that encouraged the creative enhancement that led to so many new versions of role-playing.
At first conceived of as a type of wargame, original D&D and the many new adventure or "role-playing" games that soon followed mechanically focus mostly on combat. Chivalry & Sorcery (1977), Traveler (1977) and RuneQuest (1978) were early systems that presented the players with something more than fight, loot and level up. Asking the question: "What do characters do while not adventuring in dungeons?" games such as these gave context to the imaginary lives of the PCs by discussing society and the character's place in their make-believe world. Background, social class, birth rank and family ties, grounded the character and formed connections that are then leveraged through role-play, thus adding a new social dimension to the game.
Investigative play, such as that emphasized in Call of Cthulhu (1981), added additional mechanics and brought yet another dimension to playing one's character in the game. Personality and sanity are perhaps more important in Call of Cthulhu than combat skill because many of the "monsters" are virtually beyond the reach of mankind. Solving the mystery and preventing the worst case scenario become the goal of play rather than killing monsters and taking their stuff. Knowledge becomes more powerful than weapons in such a game and connections with NPCs and setting organizations can greatly aid the players in solving mysteries.
Giving the character personality may start with a name, a background and a distinctive accent or vocal tone, but motivation, belief and connections to the social fabric of the setting are what brings the character to life as an entity distinct from the player. Rounding out the character, getting to know who they are and how they think and behave, what they value and believe to be true is getting inside your character and really playing a role. "What does your character know?" "How would your character feel about what is happening?" "What motivates your character?" These are questions that a role-player asks.
A distinctive physical description and a unique voice may signal the character is an individual much like their name does. It is often entertaining to see the character depicted and hear the voice of the character when they speak, but this is entertainment, and not the same as role-playing. We actual humans are more than our name, hairstyle, dress and accent...that is just the superficial aspects that are immediately apparent to any casual acquaintance. Getting to know a person means learning what they are all about, their story, as it were. Real people are complex interactions of their history, beliefs, values, motivations and more.
Whether the character develops as he/she/they are played or appears full blown in the author's mind, knowing who they are and how to portray them through play adds an extra dimension to the shared fiction and helps the players discover a game within the game. Is it necessary in order to have fun? No, of course not. Many have enjoyed the hack-n-slash element of the hobby at its most basic tactical combat level. Killing make-believe monsters is fun. Grabbing imaginary loot and leveling one's character is rewarding play. But the potential exists for the game to be so much more.