DM's often have lists of problem players but one who I never see talked about is something I call the wallflower. This is the player who seems to be plagued by stage fright that they just can't seem to get over. They always just go along with the group quietly. They always wait to be prompted to do things, never taking initiative. And oh gods, if they are EVER in a position where they have to make a decision, they will delegate that decision to someone, ANYONE else, immediately.
I do sympathize with them as someone who was incredibly timid all through school. You want to be more involved but you're terrified you're going to do something wrong and make a fool of yourself.
To said players, I will tell you right now. Making a fool of yourself creates some of the best DnD stories. You're absolutely NOT doing something wrong when that happens. This is not a job interview. It's you and your friends having some fun and if you get a laugh out of people, that's a success. Not a failure.
To DM's, this might help you as it has helped my players often times. Make heavy use of downtime between adventures. Get your players to answer the question "what do you do when you're not adventuring?" In an earlier campaign, we had a cleric who hadn't really characterized herself that well. That all changed when I talked with this player out of game about her goals and such and we ended up coming up with the idea of her starting a campaign to have a temple of Maou added to the city. She ran a petition, got enough public interest. The city council arranged a meeting where she was brought before a group of investors who had all manner of questions for her to answer about her plan. He was actually legitimately nervous about this which is good, it meant he was now invested in his character. In the end, the project was approved and things rolled on from there. He's been a much more active player since then. Just had to crack that ice a little bit.
Had a similar experience with an alchemist in the group who hadn't done much to distinguish his character. Once again, we got together and talked about his character. She would end up going on to found an apothecary, hired employees and in the current campaign I'm running, it's now become a megacorp still run by her as an NPC. This made her character more distinct too and much more fun. She did things that were unnerving and strange but were ultimately serving the cause of good. It was like "What if a mad scientist was playing for the other team?"
And yes, this was the alchemist:
And this was the cleric:
Bottom line: To shy players, don't let doubts waste good ideas. Got an idea? Pull the trigger! Even if you fail, it beats coasting on mediocrity any day. Just trust me on that. This is a game, no real lives are at stake here so just have fun, whether your ideas work on not. At least you'll have a story to tell either way.