what exactly is the difficulty here?
They weren't really; not yet. To those who loved John Kennedy the transition of power seemed needlessly cruel. Certainly it was harsh. It couldn't be otherwise; the brutal murder had guaranteed that. Consolidating the two groups on one airplane was to prove extremely unfortunate, and aspects of Johnson's behavior in shock may have proved exacerbating, but the difficulty there was largely one of manners and mannerisms. Johnson was not himself that afternoon. 】
what does "of manners and mannerisms"mean?more context:
【In Dallas the national interest required strength, not elegance, and it is arguable that Johnson, far from taking over too quickly, did not take over quickly enough. The United States needed a President, yet neither he nor his advisers had fully grasped the fact of Kennedy's death. Valenti spoke for the majority of them when he burst into 26000's stateroom and said, "I got here as quick as I could, Mr. Vice President."】