The other 2 answers are partially correct. I've seen the movie and the definitions of a "Class Act" given by Taylor & Chad are correct, but what Al Pacino's character means when he says, "The only class in this act is sitting next to me..." is that the only person of class (or integrity, quality, etc..) in this room is Charlie (because Charlie was sitting next to him). Hope that clarifies. :)

It is referring to a previous statement (I assume) describing them as a "class act", when you call somebody or something a "class act" you are suggesting that they are classy, or high quality. he's suggesting that, of the two of them, only the boy is "bringing class to the act", ie, he is suggesting that the boy is what is to be regarded highly, not himself.

It's a play on words. "A class act" refers to someone or something is a model of excellence. I haven't seen this movie, but I would assume that by saying "the only class in this act", the speaker is saying "the important thing here is...."

Thank you, Elizabeth.

There are some good answers. I will just add: "Class act" is an expression consisting of two nouns, but the first noun is functioning in this case as an adjective. We normally say "class act". We do not normally say "the only class in this act". The scriptwriter was just trying to be clever by changing the expression. Here is a video to explain about nouns that are sometimes used as though they were adjectives (like in the expression "class act"). www.youtube.com/watch?v=mnhYJ4w0HqU