Get the accent. Stefano is originally from Italy. He has a thick Italian accent that you can easily distinguish. He also has a penchant for saying people's full name's as opposed to what's commonly used. For example, instead of saying Sami and Lexi, he would say Samantha and Alexandra.
Have a laugh. Stefano has a very sinister laugh that he lets out when he has gotten away with something. It similar to what a villain in an old horror movie might sound like. Think of the exaggerated "buwahaha!"
Wear a suit. Stefano always wears suits. He has claim to a billion dollar family fortune, so he can only be seen in the best. Full three piece suits in black, gray and navy blue are his favorites. He never wears light colors as they don't go with the persona of the character.
Be arrogant. The main reason Stefano acts the way he does is because he believes the world is his. He thinks everything belongs to him and if he wants something, he should have it. If people tell him no, he laughs in their face. Stefano's arrogance is what makes him a dangerous opponent to anyone who is against him.