celebrity often wears a disguise when he wishes to go unnoticed. In order to make sure that nobody recognises him, he makes a conscious attempt to alter his looks. He may decide to sport a wig, wear the kind of clothes that he normally doesn't, etc. By changing his appearance, he is pretending to be someone that he is not. People usually wear a disguise in order to fool or trick others.
*In many of our old movies, the King always used to disguise himself as a beggar.
The word ‘guise' has a negative connotation as well: it suggests that the individual is pretending to be something that he is not. In this case, he doesn't necessarily alter his appearance; he changes his manner or behaviour. He puts on a front.
*She got him to tell her all his secrets under the guise of friendship.