Ash has absolutely no idea he's famous. If you recognize him from something he'll be like "me??? you paid attention to me????? you remembered me??????? of all people???" And he'll try to be cool about it (and maybe use it to annoy his friends, depending on who he's traveling with at the moment) but inwardly he'll be deeply touched. He has no idea that he's famous, and that's why people recognize him. He's one of the strongest trainers in the world and he's at least somewhat aware of that, at least, but he has not made the connection that he might be famous as a result.
Pikachu is EXTREMELY aware that he's famous. He will milk that for all it's worth. Ash may thrive in the spotlight but Pikachu is a glutton for it. He'll make a big show of acting the same way Ash does but it is A LIE. TOTALLY AND COMPLETELY. As long as he has no other responsibilities Pikachu can and will do all he can to get people to shower him with praise and affection. He doesn't even need it to sustain his ego or anything, he can function just fine without credit or acknowledgement, he's just a greedy little rat. Love that for him.