If you must criticise an author to his face then be sure to do it at the end of the conversation. The killing rage begins with the first unfavourable comment, beyond that point the writer has ceased to listen to a word you're saying and is instead imagining how good it would feel to rip your arm off and beat you to death with the wet end.
When dealing with an enraged author never, ever, state that you have no intention of buying their next book. The only thing keeping you alive is the squeaky little voice in the author's head that reminds him/her that dead people don't buy books... It's a very tiny little voice at the best of times.
Before making a criticism ensure that you are not alone, standing by a ship's railing, pool of lava or acid bath or conveniently far from any CCTV coverage. Writer's are very weakly socialised and it's only fear of arrest and prison that is holding them back.
Next week - advice for writers on how to vent in safety.
(1) 'When Authors Attack' presumably.