Professionalism is two things. Firstly it is always doing your utmost to ensure a safe, legal, efficient flight that meets the needs and expectations of your customers. In the case of passengers that includes maintaining their physical and emotional comfort. And much of that is achieved through the second part of professionalism, always maintaining the appearance of someone who will fulfill the first part.
The first part is achieved by following checklists, learning everything you can about your airplane, insisting that things are done right even when it is inconvenient, keeping current with and obeying all the regulations and policies, always flying with the precision that would be demanded of you in the most extreme emergency, identifying your weaknesses and working to strengthen them, and reporting for duty sober, fed, rested and fit. The second part is achieved with a recent haircut, white shirt, straight tie, and whatever other accoutrement your company has decided the passengers need to see to be convinced that you can fly; treating your customers and coworkers with respect; not being identifiable in any way as a commercial pilot when you celebrate your friend's birthday with half a dozen martinis, and not going out for Hallowe'en dressed as the World Trade Centre with half an airplane stuck to the side of your head.
I find it interesting that these two separate requirements are covered by the same name, and treated as one. You could call it being professional versus maintaining a professional appearance, but part of being professional is maintaining a professional appearance. Some people are better at one than the other. Some people fly safely, stay sharp and work well with everyone except management, because they find the second part to be utter bullshit. I don't think one can maintain the façade of the professional appearance for long without having something underneath it. But then I've never been a good liar.