A spiel is a prepared speech. It's designed either to recommend a product, or to present the merits of some idea, scheme or investment opportunity.
A spiel is glib, a spiel is plausible, and a spiel sends every antenna of any averagely cynical listener fizzing with suspicion.
A spiel may not be insincere - that may be the best vegetable box/loft insulation/county councillor in the country - but the fact that the spiel has been recited so often will mean that the speaker's brain is not fully engaged with the words that are issuing from his mouth. And this will be sadly obvious.
Have pity on the poor salesman. An actor only recites his stuff half a dozen times a week. A priest probably not even so much as that. But a salesman might be declaring the delights of Soapo a dozen times an hour.
And often it'll be rejected a dozen times an hour, too.
It must be utter drudgery.
Which is one reason why the origin of the word spiel is so odd.
Nuts and Bolts: spiel. The word Spiel is German for game. Mind you, it also means play, as in the theatre, which does make a bit more sense.