One of the things that really great writers do is they instinctively set roles for each of the characters in their stories. Roles are basically the set of guidelines to what the character is, what they do, what they don't do, what they won't do, and so on. In some ways this is part of the character themselves, and some of it is part of the hierarchy of characters within a group.
One of the reasons behind having roles is you don't want one "master of all trades" type character who can do anything and never fails. That's an excessively powerful character and should be avoided in any and all writing you do, save many for parody. Applying roles to your characters also helps you stay within the golden rule of character creation, which is that for every strength must have an offsetting weakness. The greater the strength, the more crippling the weakness.
By following these rules you will create more believable characters. Whether they're vibrant and likable is a whole other story. But at least you won't have a super villain (or hero) who's so ridiculously powerful that he blows up entire planets every time he sneezes.
One good example of roles comes from Lord of the Rings. It's also a good example of character balance. Sauron was the most powerful villain in Lord of the Rings. In fact, he was so strong that entire armies couldn't defeat him in single combat. If you remember the opening of the series, he blew through the human army like a gigantic armor clad weed whacker. That's a character who, by himself, is too powerful. However, Tolkien was smart enough to seek proper character balance by using Sauron's ring as his ultimate most powerful weakness.
But if big, scary, immortal bad guys is a bit too big for your mind to grasp, let's take a look at some of the people from the Fellowship and the various roles each of them had.
Aragorn - Leader, albeit a reluctant one at first. He's also bodyguard to Frodo for some time. Later he becomes king. Frodo - Ring bearer and eventual hero of the story. Samewise - Frodo's support. Without him, Frodo wouldn't have made it to Rivendell, let alone Mt. Doom. Gandolf - Guide and advisor. Later on he becomes a warrior against Sauron.
As you can see, each one of the four I listed carried an important role in the book, and even changed roles as they went along. Honestly, that's fine. In fact, I encourage writers to change the roles of their characters from time to time, as it helps to prevent monotony and the real risk of characters getting stuck in one role or overly typecast, making it hard to break them out of that later on.
Yet at the same time having roles is good for the story, as each person can generalize in a number of things, but specialize in just one or two. For example, in my latest series, the "Offworld Chronicles", there's a six person team at the center of the story who drive the plot forward from beginning to end. It starts out with a character named "Simon" who starts out the race alone, but ends up picking up five other hiking companions completely by chance.
Those five in turn benefit him greatly throughout the story and act as very powerful supporting characters for him. They each also have a special ability or skill that makes them a much needed and special part of the group. Simon is the impromptu leader, more because he was tossed into the roll than because he accepted it, or took control. He's also a computer programmer, so he acts as the "techie" of the group. That doesn't play out much at the beginning, but it does come into play later on.
Towards the beginning his dual skills of woodsmanship and survivalist are used almost exclusively, but those slowly give way to become a joint partner with his more technical side. This plays an important role on keeping the others in his team alive. Alex is the group's military expert and guide, and is thus important to the team because he helps them stay safe, and guides them through the race they're in since he's been through it numerous times.
Trevor is a street smart kid from lower Chicago who acts as the muscle and energy of the group. He's lived around the rougher types and knows how to handle himself well among them, and with various weapons. He often acts as Alex's combat support partner. Aria is the only female of the group, and balances things out a bit by providing a female perspective on things, as well as her unique ability of observation and discernment. In other words, she can see things and understand situations long before anyone else can.
I also added two aliens to the group for the purpose of variety, and as representatives of the alien contingent that exists on the planet. Since events take place on an alien world far off in the galaxy, where other aliens exist, this is highly important. Normally you see mostly humans throughout the story, as the aliens tend to either not make it very far in the race before being taken out of action, or they tend to keep to their own kind, much as humans do.
Birash, a werewolf like alien, adds wisdom and insight to the group, providing somewhat of a "guru" role, as well as muscle whenever danger is near. He's also a very adept hunter and can help provide the team with fresh game from time to time to pad their supplies as they travel. Yurg is his counterpart. He's a green striped tiger who at first hates Birash (his race lives in the same planet as Birash, but the two races have a bitter rivalry that carries over even to this world) but later befriends him. His role within the group doesn't materialize right away (which is fine), but eventually becomes both hunter, and scout.
While his role might not seem important, Yurg has an incredible nose, even better than Birash's. His eyesight in low light is also exceptional, and thus proves to be a huge asset to the group. As you can see, each of the characters has a specific skillset, or several of them, and they're each used for different things along the way. They also mature throughout the story, with Trevor and Aria at first hating each other, and then slowly falling in love.
That change in personal status from reluctant companion to friend to fiancée also changes the things that Trevor is responsible for, because he now has an additional reason to do what he does best. All though this we see their roles being exercised, exchanged at times, and even upgraded to some extent, and in the end it acts as a key driving force that motivates the plot and moves it forward.
You also have to consider that, without roles, the characters lose the specificity that's needed to drive them and the story forward. If your character is sorta blah because they don't have specific roles and characteristics that play into the story, you'll find that your plot will die an early and painful death and the story itself will fall apart. So always be mindful of the rules your characters play and be sure to exercise them in the right way that best benefits them, and the story at large.