I have no idea how writers embody boring characters, it just seems an impossibility to me. Quite frankly if I think about it those who use boring characters just seem lazy and probably have no emotional investment in their story(with the exception of The Secret Life of Walter Mitty, but he was not boring persé).
Well there are several ways a character could be dubbed "Bland". Keep in mind, bland mains aren't necessarily boring, but mainly empty/lacking/uninspiring/and yes boring is one as well. The issue I see (at least characters I hate the most) are the ones that are the following:
1) Perfect. This isn't inherently bad, but having a perfect character that can do no wrong/isn't bad at anything really makes the main come across as unreal/unrelatable/and basically "Higher than thou". If anything it's a turn off.
2) Lacking in motive. Now I'm not opposed to this if the character starts this way and gradually has reason to, but it really annoys me when this so called "Main" does it without caring or at least showing incentive or reason. It makes the story sound like a filler and waste of time to the pro and is often a reflection on the reader's thoughts.
3) No emotions/internal reasoning. By emotions I mean the reaction to things. I hate it when a character is affected in one time, then shrugs and is back to normal without any reason to nor any reference back to it. Its like he/she pulls off a band aid from a minor bruise. Yes it hurts, but it goes away and the bruise is gone. It's not like it's a real cut which stays for a while and the whole point feels wasted.
4) Without cause/effect/broad mind. Characters that don't feel the world or others around them are pretty annoying. This is one of the reasons I really got annoyed with Yuya Sakaki form Arc V. While I love the story's direction his striving to entertain slowly became annoying and somewhat normalized. He never thinks there's is more to the world solution besides "smiles". I can understand that to a degree but a person that refuses to see and grasp the world around them looks really self focused and lacking in understanding.
5) WTF reasoning. This one realllly gets on my nerves. When a horrible thing happens and the villain is at the persons feet and they just do the most unreal/lack of reasoning/illogical/stupid thing and don't seem to give a care about the weight of the actions. I can maybe see past this if the intention was to prove a point or not sink as low, but it's dumb and downright insulting for someone to be so robotic as to not feel anything when probably the bane of their existence or whatever is there and they say. "I win you lose, the end". Where is the reasoning/human reasoning/logic? It's sacrificed for keeping the character "Good" or "strong" in their beliefs. Ugh
But yeah, these things annoy the heck out of me. I try to write characters who are flawed and not on either extreme of the ideals spectrum. Or heck, just showing a character progress or develop is an amazing thing for a story too. While I can understand if my characters don't seem that way, it isn't always about showing your character's inner side 24/7. If you know how to space it out and time right, you can get the reader in the upright position at the main's true feelings on the matter come out.
Sorry for the long write, but to sum it up in one sentence I guess it's like this. (Ahem)
"A character that has no attributes or realism about him, along with a mediocre at best persona, wit, and global thinking, and only goes through without grasping the weight of his actions upon others while being mentally sound and capable, is the dictionary definition of "Monotonous, dull, uninteresting, and completely without relational potential."