Possible Clues:

• A bit too articulate, perhaps
• A little too slick
• Printing libellous piece that's plausible
• Smooth-talking, plausible
• Fast-talking major holding line up
• Well-spoken but insincere
• Breezily unconcerned
• Pat of butter in large glass, just starting to turn
• Insincere, easily said
• Fluent but insincere; plausible