Prognosticates, when one chooses the players in advance (9)
I believe the answer is:
forecasts
'prognosticates' is the definition.
('forecast' can be a synonym of 'prognosticate')
'players in advance' is the wordplay.
'players' becomes 'cast' (group of actors).
'in' is an insertion indicator.
'advance' becomes 'fores' (I can't justify this - if you can you should give a lot more credence to this answer).
'cast' inserted into 'fores' is 'FORECASTS'.
'when one chooses the' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Another definition for forecasts that I've seen is " Estimates of future weather".)