The sooner u let the other person know, the sooner and better he/she understands what you think. Diplomacy is understood as acceptance. The desire to appear nice and courteous is often mistaken, which can be a dangerous thing.
For example, never be diplomatic about
- Your teammate fraxing away to glory (Tough one)
- Friends not paying you back on time :-))
- Your girlfriend flirting with the colleague from office :-)
- Your boss showing differential treatment
There is a very thin line being being diplomatic and being Hippocratic. sometimes its not worth taking the risk, blast it all out. Atleast you don't leave the other person guessing and you don't regret the missed opportunity later. Lets just say Truth sometimes stinks.