Illusions are a game we play
It took a long time to convince Belle that it needed to be done. After he was placed in the cell she came to find him, spending days listening from the shadows of the castle until finally she heard mention of the underground dungeon. She immediately tried to break him out until he explained, several times, how it was important to the curse that he be there. The royals had to believe him powerless, and only then would they speak with him, deal with him. The Savior's name was his primary reason for being there. He'd planned for it to be the spark that would reawaken him to his true self in the new world, and Snow White would only reveal it if she believed him to be powerless. Of course mentioning his 'true self' meant he then had to explain to Belle what the curse entailed. When he had finally convinced her to stop trying to free him, he got her to sit down so he could explain this new world to her.
No one would remember their true lives, save Regina, for twenty-eight years. They would be trapped in time, unaging, waiting for the Savior (Snow White and Charming's True Love child) to come and set them free. No one would be aware of the curse or their true selves until the curse was broken. Then he explained how he had made the curse, and manipulated Regina into a position where she could cast it. Belle was understandably horrified, and she didn't come back to visit him for a further three days, during which he was visited by a gloating Regina. When Belle finally returned he explained why he needed the curse, and she was glad he had at least done it for a legitimate reason, and not because he thought it would be fun. Then he told her stories, so many stories, about his Baelfire. She didn't forgive him, couldn't, when he refused to give up the dagger, but she understood his actions a little better.
To be honest she was glad he had a reason for rejecting her, since he needed his magic to find his son, but she wished he would have told her that before. And to keep her free will from her was a cruel thing. She might still have helped with the curse, if he hadn't made it quite so awful for people. Twenty-eight years in a state of purgatory... And to have manipulated Regina into such a thing was truly evil. She could understand now how hard it must have been to fight the magic within him, and the way it lured you into the darkness, but to force others onto such a path, to awaken the cruelties in their heart and make it so they couldn't resist... She didn't talk to him for a long time after that, either.
A few weeks into Rumple's stint in the dungeon, it became apparent they were going to have to come up with a new method of communication. When Belle visited, invisible as always, Rumple would sit in a corner of his cell and whisper to her as discretely as he could. But he was getting tired keeping up the appearances of the Dark One (who rarely slept), and was having trouble dealing with the vile food that was provided. She suggested using illusionary magic to give him the chance to rest, and a few days after her suggestion she came back with a plan. Rumple agreed that it might work, and so she began to put it in place.
He watched as Belle constructed, with her magic, fake bars to the cage a little further down the dark tunnel. They did not block magic, since they were not truly there, so she could pass through them to the real bars (which could not be seen past the fake ones) in order to talk to Rumplestiltskin. She also created a Rumple-Puppet that, with a lot of effort, could pass into and out of the real cell, and could also touch the fake bars. As an unliving entity it had the energy to leap about and terrify the guards at all hours of the day, and it meant the real Rumplestiltskin could no longer seen. Belle dropped the glamour she had placed on him so he had his human skin once more, and also the healing spell on his leg, so she might have more energy to fuel the illusion. At meal times she would bring real food, breads and meats and actual fresh water, as well as sometimes tea and biscuits, so that he would not have to feast on worms and slime.
He was still miserable though, because every day he would be subjected to the guard's taunts and glares, and Belle's haunted expression whenever she came to sit with him. He saw her less often now, for perhaps an hour or so a day in comparison to watching her flit about the Dark Castle all the time, and found himself immeasurably lonely. Their conversations were hurried and brief, subdued over a rushed meal she had brought him. Sometimes they were cut short by a visit from the Royals, or even the Blue Fairy. Belle always avoided the tunnel at those times, lest the invasive gnat learn the Dark One was traipsing about in the darkness. Her refusal to bow down to the dark nature of his (now her) curse meant she was more tired than he would have been doing the same magic, since she battled herself constantly; it didn't help that she was new to magic altogether. In his Belle's expression, masked by the fear, was also an immense sadness. He told himself it was because she was lonely; because she was cursed; because she missed her father (banned by the dagger to speak with him), but he rather wished he had found the chance to tell her he loved her back.