Etiquette when talking to a researcher whose work you do not think highly of What do academicians need to keep in mind when they are conversing with a fellow researcher whom they do not think highly of? I was reminded of A Beautiful Mind, where Nash (Crowe) says the following:


  I imagine you're getting quite used to miscalculation. I've read your preprints... both of them. The one on Nazi ciphers... and the other one on nonlinear equations... and I am supremely confident that there is not a single seminal or innovative idea in either one of them.


This is as forthright as it could possibly get! Suppose there is a situation where you have read a paper in some detail, and are sure that it is not just worth its salt. If you happen to strike a conversation with the author about the work itself, how do you go about it? How do tactful researchers react to this situation?
Be polite. There's nothing to be gained by being a jerk. It's funny in the movies, but do people really say things like that in real life?
Be diplomatic. State initially that you do not fully understand the approach or the motivation, and then after some discussion state that you do not fully agree with the approach taken.
Be succinct. Point out clearly what you see as being the problem with the paper.
Be constructive. Indicate places where you think the paper/research could be improved, with concrete suggestions of how you would make the improvements. 


Alternatively,


Avoid the topic. Talk about the football scores instead. What about those Bears?