In many fanfictions, Naruto might hide his pain by putting on a big smile. He's always been the type to try and be positive in the face of his friends. He could also distract others with his goofy antics, like making a joke or doing something silly to shift the focus away from his inner turmoil.
Naruto could be afraid of being seen as weak. In the ninja world, weakness is often frowned upon. He has a reputation to uphold, especially in front of his family. He doesn't want them to think that he can't handle his problems, so he hides his pain. He also might be trying to be strong for his family. If they see him in pain, it could affect their morale. So, he pretends that everything is okay to keep their spirits high and to give them the impression that they can rely on him no matter what.
Perhaps he has hidden the full extent of his Sage Mode abilities. He might be able to stay in Sage Mode for a much longer time than he has shown so far, or have some other undisclosed powers within that mode, like the ability to sense enemies from much greater distances.
One interesting plot could be Merlin hiding his pain from Arthur because he doesn't want to seem weak. He might endure the taunts of the knights in silence while secretly using his magic to heal his internal wounds. Another could be Merlin hiding his pain over his mother's absence or the fact that he has such a huge responsibility on his shoulders with his destiny to protect Arthur and Camelot.
The main character is Merlin, of course. He is the focus as he is the one with the hidden pain. Arthur Pendragon is a key figure too. Since Merlin is Arthur's manservant, Arthur's actions, words, or even his ignorance of Merlin's true self can be sources of Merlin's pain. Sometimes, the Great Dragon can also be involved. The dragon might be aware of Merlin's pain and offer some cryptic advice that Merlin has to decipher while still hiding his pain from others.
In some fanfictions, Naruto's pain might manifest through his actions. For example, he could become withdrawn and isolate himself from his friends. This is because Naruto has always been a social person, so any deviation from his normal behavior could be a sign of his internal pain.
One possible cause could be his sense of failure. Naruto always wants to protect his friends and village, so if he fails in a mission or is unable to save someone, it can cause him great pain.
Sasuke could be one of the people who help him. Despite their past differences, they have a deep bond. Sasuke has his own skills and knowledge that could be useful in Naruto's situation, like his Sharingan abilities which could help in detecting enemies while Naruto is hiding his face.
He might use some special jutsu or seals. For example, he could use a sealing technique to suppress the signs of his bloodline. This way, others wouldn't be able to detect it through normal means like sensing chakra.