There is a huge number of things that could be included in an answer to this question, but as I do not know what your general process is like as well as for the sake of brevity I will just delve into one exercise that has worked well for me.
Start by deciding what kind of personality you'd like your character to express
This may seem a little obvious (possibly uselessly so), but it's a huge step in getting out of a rut of any particular character archetype. A character is much more strongly defined by what they are than what they aren't. From there you can give your character goals, aspirations, and experiences that will enforce that personality naturally even as the character grows over the course of a campaign.
For the purposes of this example I'll assume that you're going for an upbeat, off the cuff adventurer who prefers not to drag the mood down. Obviously that's just a quick flip of the descriptors you used but it'll be a good starting point. Now, while you could go just with that decision and run with it, doing such would likely end in you drifting to your default type as your character gets jaded by the complications and trials of a campaign so what I like to do next is to define the motivations and traits behind the aspects of the personality I'd like to play.
Lets start with a character being upbeat or at very least not falling to general grumpiness. There are a number of ways that you can take a trait like this. One example is innocence. A character with a childlike innocence is unlikely to understand the full gravity of situations that may spur another to a darker mindset. If you prefer to twist it a bit, you can carry that innocence out to disturbing levels, but you don't need to to keep the upbeat personality. Being upbeat doesn't mean that the character can't get sad or have moments of depression but they should be the type to be ready to move on either to a plan of action or to whatever comes next. Another example is to have a character who is generally at peace with the world and it's complications, such as a holy person who understands that perseverance and good cheer are things required even in the darkest times in order to light the way for those both within and without their faith.
To get away from by-the-book style characters, you simply need to have them be more comfortable improvising or with some inclination against "the book" due to prior experience, belief, or general mistrust. Perhaps their ability to work on the fly has helped them in previous situations and they are extremely confident in their ability to continue to do so. Perhaps they were actively hindered by the standard way of doing things and only were able to fix things by going outside such (or maybe they didn't and wish that they had in hindsight) If it helps you can imagine situations dealing with by-the-book characters or situations and figure out how your character would react in such a way that the result reenforces your character's beliefs, rather than hampering them.
Finally not wanting to drag the mood down speaks of an interesting mindset. While some of this touches on the upbeat part; wanting explicitly to buoy the mood of those around them speaks of some goal. This could be the trait of a born politician or noble who knows that happy people are not only more tractable, but also more effective. Likewise it could be the trait of someone who is fond of their traveling companions and takes some level of responsibility for their mental well-being. It could even be that the character is entirely motivated by the simple selfish desire to maintain their own happiness by not being surrounded by depressing people. All of these speak to different characters and there are many, many other ways that such could be taken.
The more reasons and situations you can figure out ahead of time the better foundation you'll have but don't forget that your character's personality will continue to take shape as you play. If you choose to, you could turn any or all of the above character pieces into the exact taciturn character you're hoping to avoid through the trials of your campaign, but (baring in game magic personality shenanigans) only you get to choose how circumstances affect your character's outlook and personality. Even abject tragedy can in the end bolster a character's upbeat spirit without seeming fake or forced. It could be that the moment reminds them to make the most of what they have, or it could be that they simply refuse to be broken even by the worst the universe has to offer.
At the end of the day, it simply comes down to what kind of character you wish to play and then figuring out the "How"s and "Why"s that build up to that person naturally.