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 am not sure about this - if you are sure you should give a lot more credence to this answer).
'cast' inserted into 'fores' is 'FORECASTS'.
'when one chooses the' acts as a link.
I am not very happy about this link. 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".)