Playing a premade character is very different than playing a character that you’ve made yourself. There are a number of reasons to play premade characters and a couple different types of premade characters that you can encounter.
The first type of premade is the one that the game itself provides. Many games have sample characters that a starting player can play in campaign packs, starter packs, or even core rule books sometimes. A variation of this is the premade characters available for official games at conventions to streamline play. They tend to be very basic, have a standard array of skills, and follow set patterns of character creation.
The other type of premade is a character that someone you are playing with, or your GM, has made for you. These characters tend to be a bit more interesting than their game provided counter parts. For one thing, if your friend/GM is making you a character, it’s usually more personalized, whether you’re involved in the process to the point of them checking in with you for small details, or it’s just them working off of your initial text reading “Goblin bard high seduction”. Even if they’re just a character that the DM cooked up as an extra, they’re probably more interesting and better optimized than a game standard Premade.
Premades are useful: they’re good for a very beginner player, (especially if it’s a group of all beginners); they’re good for one shots where you don’t want to spend time making a character; and they’re useful for people who (like me, one memorable saturday afternoon) arrive 9 hours late to the first session and miss character creation, or just don’t have time to create one pre-session.
Premades present a couple different challenges. Not having made the character means you don’t know the character’s abilities, skills, or equipment, and, from a role playing standpoint, you have to jump into a character that you haven’t been able to think about before. You don’t get to customize the character, so they may not feel like yours, and you probably don’t have as much invested in them.
When you pick up a premade character, take the time to read through it, even if you are experienced in the system. Make notes or highlight important things such as initiative bonus, attacks/abilities you think you want to use, number of spells, and Armor bonuses. If you are new the system, ask someone with experience what will be the most important or most commonly used roll, and make sure you know that stat. If the DM has provided the character, ask them if there is anything important you should know.
It can be easy for a premade character to get boring: they may have one optimized attack that you find yourself doing over and over again, or they may not work well in the situation you find yourself in (for example, a LG Paladin in temple robbing expedition). If this happens in a campaign longer than a one shot, ask the DM if you can tweak the character! Changing a simple thing like alignment or one feat to better enhance your experience as a player is usually no big deal, and can make the difference between staying in a campaign or quitting. For a longer campaign, you might be able to change something more drastic such as a cleric domain if you don’t want to just create a completely new character. I’ve been in at least one game where someone was attached to the personality of their character, but rewrote the build from scratch to better suit their needs and playing style.
Role playing a pre-made:
If you want to focus on the role play aspect of a pre-made, it can be harder. For a quick character: a guest, or a one shot, or a two-fer, you don’t have to write them a 5 page backstory (but if you do, good for you). Especially if you are picking the character up at the session itself, ready to play, sight unseen. Pick one or two aspects of their personality you think are important to play, based on the data of the character sheet. Unusually high or low stat? Turn it into part of their personality. Do they have a god? Make the character pious (or atheistic, if they don’t!). Is there an unusual skill they have? Make up a reason why they have it, and sew hints of that in your RP. If they’re and unusual race/class combo, come up with a quick history as to why! If any other other detail of their race, alignment, equipment, or skills strikes a spark of inspiration, use it. Maybe your elven wizard had prestidigitation, so you decide they’re a germaphobe. Or your goliath barbarian has an unusual number of light casting items, so you decide she’s afraid of the dark. Pick a couple details of their persona, and play based on them. You don’t need to put a lot of thought into it, and the character should evolve from there. Even if the details you’ve picked never come up, they help give you a sense of who that character is.
If you are working with a longer session, you should have time to create as intricate a backstory or personality as you feel you need. If you are coming blind into the first session, you can still do a quick assessment of personality and build from there, adding onto their backstory as you go, or writing it when you have time.